Can you raid a 90 and 120 ssd together?

Yes but the raid would be 90gb and you would see very little speed increase.

Kinda need to be identical for raid 1, 0, 0+1, 10, 5, 6. If you want to set them up as J.B.O.D (Just a Bunch of Discs) they don't need to be identical, what'll happen then is your computer will see it as one 210Gb disc (or whatever your discs add up to). Downside to that is if one disc fails you lose all your data.

Technically, yeah, but there's not much point in RAIDing an SSD. They're more than fast enough as it is.