...a song with a duet of voices (male and female specifically) and at least two other intruments? Is it an Aria? Or just a duet, or ballad? Ballads are calmer and sweeter and such, so I think it'd be a ballad?

It isn't an Aria, since that is for solo vioce with a tpye of accomp.

I guess just a duet-but an accomp. can be there I guess.

EDIT : And it can't really be a ballad, unless you use it with it's modern definition (as you did). Other than that, originally a ballad was a song with many phrases, and a possible refrain.

Duet I'd say.

Duet I'd say.

I agree.