Your can mix certain species of shrimps. First of all some of them needs different water parameters, but there is another problem, they can interbreed.
Your right about neocaridina species, they should not be mixed. But you can keep cherry/yellow/blue pearl/snowball shrimps together with tigers/crystal red/golden bee shrimps. You can also keep babaulti species (rainbow and indian zebra) shrimps with any of mentioned above.
I have a short article about interbreeding here: Can these shrimps be kept together? It also has a chart with all common freshwater species.
Undemanding shrimps like neocaridina species (cherry shrimps) will eat almost everything. Vegetable-based fish food will work great. I feed my shrimps with spirulina flakes and algae pellets very often. I also give shrimps fresh or frozen organic vegetables and, of course, algae. Soft green algae is the best food!
Some fancy shrimps like sulawesi, tigers or crystal red shrimps need more care and there are some special food (like Mosura) made for them. Bacteria and beneficial minerals are in there.
It's very brief Shrimp keeping is very existing and there are lots of diffident aspects of it
