Richard told Jack that those touched by Jacob cannot take their own lives. Knowing from Jacob's message from Hurley's guitar case, that Sayid was a candidate, Dogen hands Jack the poison so that he can hand it to Sayid saying it will not work if Sayid does not take it willingly, of his own free will. Based on these two pieces of information, it is safe to assume that Sayid would not have died from ingesting the poison had he taken it. Is it fairly safe to assume that, him being claimed by the "darkness inside him" has changed Sayid in more ways than one, and therefore the Rules Jacob and Fake Locke live by no longer apply to him, whether he is a candidate or not.
Richard never said to Jack that those touched by Jacob cannot take their own lives. When Jack asked him why he could not die, he simply said because Jacob touched him. Because this applies to Richard, doesn't exactly mean that it applies to everyone.
Jack did, however, ignite the fuse for Richard, yet, Jack was not blown to bits. Instead, the fuse fizzled. This illustrates the need to include "the island won't let you die because you have work to do" variable into the theories of this sort. Jack and Richard didn't die because they both still have work to do. Michael could not kill himself either when he had unfinished work. Is this different?
Sayid failed the temple's "witch hunt" test
The test at the temple was to see if Sayid was either Jacob or MIB. Since he reacted like a normal human, he 'failed' the test.
Being tortured may have been a test to see if Sayid's position on destiny/fate vs. free will. By asking "why are you torturing me?" he is implying that he was not accountable for his prior actions (e.g., he was destined to be a torturer) - a response consistent with MIB's philosophy. Accepting torture without comment would indicate accepting personal responsibility for past choices he had made (consistent with Jacob's belief in free will).
Dogen and the Others are witch hunters who poison people they think don't pass the test (the DI being a good example).
Dogen blows what I suspect is metallic dust across Sayid then shocks him. I think there is a special electromagnetic property associated with claimed or infected people and these first two actions are diagnostic as stated and then the branding is a symbolic 'marking'.
Sayid was indeed submitted to a test: Dogen used a combination of electricity and that (very likely magnetised) powder to determine if Sayid was in contact (I’m not going to speculate about possession here) with the Smoke Monster. The use of the powder (which is the same substance used to repel the monster) is of the utmost importance here. We know for sure that two things can stop the monster: the grey powder/ash and the sonic/electromagnetic/microwave (still not clear on that one) Dharma fence. Also, did you notice how gently Dogen cranked his little generator? Sayid’s reaction seemed out of proportion with the brief, possibly low-voltage shock he received. That could mean one of two things: either the powder amplified the shock or his reaction to the treatment means that he was indeed “infected” by the monster.
There were two tests. One test used an electric current (either directly to deliver a shock or to generate a magnetic field). The other test used a red-hot brand. The result of the second test is interesting. The burn may have had no effect on him or he healed immediately from the wound. Sayid also states that he "clearly" failed without anyone telling him that he failed other than Lennon stating that he passed. Sayid is pretty good at reading people, but the word choice of "clearly" may imply that he knows he should've been badly burned.
With how shallow those needles went and how small they were it seemed almost like acupuncture of some sort, which is often combined with electricity. In a healthy person, this would reduce pain and restore balance to the body. Sayid is now no longer in "balance" and therefore is harmed when the needles apply the shock.
This is a show where EVERY single blow to the head causes immediate and long-term loss of consciousness, I wouldn't argue Sayid's reaction to painful stimuli too hard here, clearly the show is taking some medically-related artistic privilege here, rightfully so.
Although only remotely related - there's an uncanny similarity between how Sayid drowned then returned to life and how Charlotte Malkin drowned and then returned to life in Eko's flashback. She even states that she saw and spoke with Yemi while 'between places' (she also mentions John Locke, whom she didn't know) - which would be Eko's reason for being in Australia and also his reason for being on the flight which crashed on the same island his brothers corpse was on - all MIB's handiwork in setting up his little loophole? Was Charlotte claimed in the same manner in which Sayid was? Interesting similarity at least, though most could be due to Eko originally being the MIB but leaving the show before that could happen, so it could've been the original beginning of the loophole groundwork.
The Losties (and Jack in particular) have spent six seasons dealing with people that all seem to know more about what's going on and deciding what the Losties have to do about it. Like Ben, once again Dogen now knows best and once again he's wrong. This is the time for the Losties to step up and start demanding answers and making decisions for themselves. Jack swallowing the pill is just the beginning.
Sayid failed the test because he should not feel pain throughout the entire test. When Dogen inserted that wires, Sayid received some kind of acupuncture. The electricity and the poker could harm him, but he should feel no pain. A normal person could be afraid and scream, but Sayid screams in pain. Dogen knows how to distinguish fear and pain very well, but Lennon is not sure about this, for this reason he asked, "I just lied to him, didn't I?".
The test Dogen performed on Sayid is somewhat similar to what Rousseau did to him earlier, except that Rousseau interrogated Sayid about the whereabouts of Alex.
The red-hot poker was not a test, nor a form of torture, but rather a part of the remedy. Ben and Sayid were both taken to the spring with a gunshot wound and came back to life. Both men were saved spring's healing properties. When Sayid is laid on the gurney in Dogen's lair, we can clearly see the gunshot wound It's not bleeding but it's there. Dogen burns Sayid to seal the wound, either to prevent what got into Sayid from seeping out, or, in the case that Sayid is not "infected" yet, to prevent it from getting in.
Dogen blows the gray ash on Sayid, ash that usually is a protection, such as around Jacob's Cabin, so when Dogen blows the ash on Sayid and is shocked, he feels the pain. Same with the hot poker, if Sayid was not infected, the gray ash should have protected him and he would not have felt either of those things. Him feeling pain even though he was "protected" with the ash, proves that he is infected.
Sayid failed the test because the test was faked, a "placebo." There was no current and Dogen never actually touched him with the poker. Sayid simulated the responses he thought a normal being would exhibit.