I think night might help. It's worth a try. It depends on the cover they have to hide in. Eleocharis harigrass is OK, but not superb. Mine tend to play around between the fronds of a Rotala rotundifolia thicket and in and around Anubias sp. nana. The largest shrimp is the same size as the largest female M. praecox now so there is no problem. When the shrimp arrived from Florida, they were about 2-3cm in size.
Out of about 12 shrimp I put in there, 3 surivived and they grew large. In another no-fish aquarium, 100% surivive and I have a myriad of 1cm shrimp several weeks later.
Cherry Reds do not stand a hope in you know where. I bought a lot and they were expensive and stupidly fed them to the rainbows. An expensive meal!
They're certainly worth the effort (both rainbows and shrimp).
If the r'bows are small they'll ignore them for a while, but yea, they will chomp on small shrimp. If you have a very well planted tank some may survive- you'll select for the ones that can hide the best though, which means eventually the shrimp colony's color may fade.
I keep amanos with my madagascar r'bows with no problem and I always use cherries as clean up crews for my r'bow fry/grow out tanks.