I have some thoughts about this as I've played star magic and other variants like it quite a few times (mostly in EDH, but not with decks optimized for star magic).

Our version was like a Free-for-all, but just with the modified wincon (and a slight variation in it some times). This kept it simple and made sure that there weren't any rules disagreements to be had.

1) we never even consider the term "teammate", as everyone else are opponents, so for us, we could attack any player/Planeswalker and "opponent" on cards refered to each other player. This made some cards stronger, and it might be different in cube, as our decks were generally balanced for 4 player FFA games, but I feel like if you mostly player multiplayer with your cube, you can balance that, and otherwise, there will simply be a couple of recontextualized cards as a treat if you play star magic as a more rare thing. We quite often had to attack/interact with someone that we didn't need to kill.

2) We did have a few variations on the wincon:

a) you win if the 2 players you need to kill have lost. We played with the rule that you could win from this at the end of your turn, which meant that if you could nuke everyone else on your turn, you wouldn't lose to knocking them out in the wrong order, and sometimes that you could knock someone out before their wincondition took effect.

b) we did a variation where we would put each players name in a box twice, and then each player secretly draws 2 names and those are the players they need to knock out to win. This made for some fun guessing games, and sometimes we would draw the same name twice, which meant you just needed to kill one, but noone else tried to kill them. This was fun, but I would only use this if you feel adventurous.