I have a rainbow with my Jack. There were more, but its school eventually died off. It hangs out with the tetras.
The important part is that it does well with the Jack.
Thanks everyone for their input. I found a pair of young JD's so picked them up (the last in the tank -- one is almost certainly a female judging by the extremely iridescent gill plate, the other is a little smaller so not certain on that one yet). I've decided to give giant sailfin mollies a try as dithers; found a source from some wild types as well so I'm pretty excited about this combination. We'll see how it goes.