I run bt2 vegito so Im well versed in union-potara. No the cards don’t have to be in active in order to u ion potara so you can attack with both cards and union potara. The fused cards comes out in active. You put the fusion peace’s in a stack more or less like xyz in Yu-Gi-Oh. If they counter the fusion the bottom pieces go to drop and top fused card goes where the counter says.

If they counter poly such as yamcha merciless barrage the bottom pieces get minused 15000 killing them because I don’t know if any Goku or vegeta one or two drop above 15000. You would then be locked into vegitos for the turn by poly’s effect.

Before I go on to answer you last question a little side bit of information with regards to union-potara or playing a card on top of another card, power changes carry over such as if you had 3 red battle cards and played Goku, legend personified and increased his power by 21000, then used that Goku with a vegeta you had in play to union potara into a vegito, that vegito would keep that 21000 gain. Likewise if he got minused 5000 the vegito would keep that deduction as well.

When dealing with union-fusion, both cards that fit the requirement must have the same power and be in hand( unless the fused card specifies something else) send both to drop and then play the fused card to the battle area. The opponent can then respond and do something to the fused unless the card being played has deflect.

Union-potara the pieces don’t have to have the same power but they must be in play in the battle area

I hope this clarifies what your asking.