>>43316288Every Pokemon (aside from legionaries, unless you allow those) is fair game if you havent got a Pokemon in that area yet. My 2 exceptions are forced "gift"/"event" mons (ie Eevee in Sacred Gold/Storm Silver, Zorua in BW2, forced eggmons in any game) or your starter.
Only forced ones though, for example you can skip getting the elemental monkey in BW1. But if you opt to get it, thats your encounter for the area.
I also like to play with a mod that makes trade evos possible in games. Pic related. I'm a little split on it though, as a lot of the trade mons like Machop, Geodude, and Gastly are usually pretty easy to get so you can have a pretty broken team in gen 1 and 2.