In a big tank they will be fine. We had a group of trifasciata in with some discus at the shop a while back and they did really well. The only real issue you have is discus are carnivorous and rainbows are omnivores with a preference for plants. Make sure the rainbows get lots of plant matter in their diet and they should be fine.
You got some trifasciata Chris? Just ordered 10 myself on their way next week, beautiful fish, if you can sort the temp issues out, because I think discus temps are too high for bows, then you should be ok.
Rainbows are fine in temps of 28-30C and although they will age quicker in the warmer water, the bigger species can live for 10yrs so it won't take too much off their life. If you are really concerned about high temps then keep the tank at 26-28C.