What variety of Rainbows? The larger body type would eat them pretty fast. If they wouldn't fit in the mouth of the tropical tank fish I'd say it's better than nothing. But is it possible you can set up a separate tank?
Thrift stores, Restore, yard sales, CL
and Petco dollar sale- I think ends soon, are all cost effective options. That way you could go bare bottom for ease of cleaning and add a couple rocks and have the tank water hardness what you would want for xichlids. Add a couple rocks and some plastic plants for cover.