Do you decide before the tournament or each round which version you are using?

You decide when you build your team. You can tell which version you are using if the click before their starting click has their number in the set in the place of the damage KO = human, or if there is a Skrull symbol in the damage KO = Skrull.

Do you reveal if they are the Skrull version to your opponent before choosing BFCs?

Trait - When revealing your force, turn this character's dial to the slot before its starting position; this reveals to all players whether or not the character is a Skrull imposter. Use the character card associated with the indicated collector's number (006a or 006b), and ignore the other character card. Turn the dial to it's starting position before the game begins.

when revealing your force so i would assume before.

You reveal when placing your characters in their starting area, I believe.

I believe so. I think this was discussed in another thread. It seems that because of theme team bonusses and such that you need to reveal your team early on, but I figure most people could tell which version you are using based on who you were using them with if you had a theme. I do not recall what that other thread was called or where it was.

Just played this last night. The reveal is during the start area set up. I pulled 2 "skrulls" and a possible. Ms. Marvel, Yellowjacket (Skrull), and Dum Dum Dugan (not).

so is there 2 fig one skull one not?

No. The only difference between, say, 6a Yellowjacket and 6b Yellowjacket is the dial. The sculpt and base are identical between a and b versions (which is why you have to check the dial, as the character card instructs, to know which version is which).

So there are 65 dials and 60 sculpts in SI?

You got it.

I really can't believe they couldn't manage to at least just paint the non-skrull version a normal skin color. Probably would have cleared up some confusion, as well as making more sense....