Here's my thoughts on them. The structures you listed are often taught in textbooks but those with "were" seem a little old-fashioned. The modern trend seems to be using "wasn't" for the first and third person and "were" only for the second person just like in indicative mood. So I would advice adhering to those rules for you test (since testers are rather strict) but be more open and flexible in actual speech and writing. Similarly, the tense-specific paring of "were/weren't" and "would" vs "had/hadn't been" and "would have" are often broken in real life usage (and not because of misuse). So while those structures are likely the most commonly heard (and thus should be preferred in a test), they are not unbreakable rules. Also, what you listed might need some clarifications. "were" has traditionally been used in subjunctive mode, not just for "it" but in all cases, as in "If I were you, ...", "If you were in my shoes, ...", etc. The special case with "it" would be "if it weren't for (something)" which is equivalent to "if there were/was not (something)", as well as the corresponding past perfect version "If it hadn't been for (something)". So it seems a bit odd to list "if it weren't ..." without "for", since "were" by itself doesn't have any special relevance to "it". Also, "if it weren't for" and "if it hadn't been for" seem to be not used as much these days. There is the alternative "Without (something)" which have the same meaning while being much shorter and capable of both tenses, as in "Without your help, I would be / would have been ...".

I agree with the excellent points Tony brought up on the subject. Here's my perspective on the usage. Likewise, I recommend following the rules for your test per your teacher's instructions. A better way to look at it is to consider the time frame in each part of these two structures. The first one is what we would call the "Second conditional sentences," which are used to describe make-up situations with a present or future time frame. If I inherited a million dollars, I would travel around the world. If it rained tomorrow, people would dance in the street. Although the condition clause in the first part of the sentence uses the past tense, it is more of a hypothetical condition that does not refer to any time frame. The consequence of the second part expresses the outcome referring to the future time frame. The second one is what we would call the "Third conditional sentences," which are used to refer to hypothetical situations in a past time frame. If you had called me, I would have come. I would have left earlier if you told me you needed a ride. Both the conditional clause and the consequence refer to the time frame in the past. If we go back to your question about using a condition clause with a subjunctive from the former and a consequence main clause from the latter, there needs to be a match in the usage. We could not express our intention for the past event if the condition were merely set up with no implication of referring to any time frame. Nonetheless, the following sentences with mixed conditional make sense with some thoughts to ponder about the time frame. If you had done your job properly, we wouldn't be in this mess now. The conditional 3 (past) + the consequence 2(present) If we were soldiers, we wouldn't have done it like that. The conditional 2 (past, but it is not expressed as being limited to the past) + consequence 3 (past)