according to this website you can
http/www.peteducation.com/article.cfm?c=16+1911+1951&aid=2519
Discus are relatively docile, and so are rainbows - and rainbows are quick and can generally stay out of trouble no matter what else is in the tank
Discus prefer higher temperature than the rainbowfish would like, if you can reach a happy medium, then they should be fine together, maximum temps for boesemani to be happy would be 27c, but they prefer around 24c.