never kept festivum but I will say I probably wouldn't keep them with a rainbow cichlid in a 35 gallon tank (depending on the dimensions). I've heard from several people that rainbows can actually get somewhat aggressive as they get older, but even if aggression wasn't the issue rainbows can also get somewhat large. I've heard of 6" easily with possible potential to reach 8" for males. Now keep in mind this info comes from experiences of others' that I've read because my experience with rainbows is very limited.
perhaps something a bit smaller like rams, appistos or even a keyhole would be better. you should also be able to do a nice school of a larger tetra, rasbora or non-nippy barb species.