other players take damage from sunfire's ability which states that it should instead heal allied players.

This is still an issue using FTBTeams. It does not use the vanilla scoreboard team system so using scoreboard does not work. The /team command also requires OP permissions so you cannot easily add people to a scoreboard team on larger/public servers.

Yeah I am having this issue too, I have been trying to figure out a fix because even if Partied it counts all players as enemies.