Rainbows can really go with a lot of fish tbh as long as they can't be eaten I'd imagine it's a playground. Ive personally kept some with amazon/spotted congo puffers and everyone was happy and no fins were nipped along with bichirs, rainbow sharks and flag tail catfish. Really just get a school of them and you'll most likely be fine, but don't be afraid to do some research online. Also what tank size would you prefer that's a big part too for tank mates.
I have dwarf neon rainbow in 180 litres of slightly hard water (8°H, pH 8). I bought nine and lost 3 in quarantine (well one is still there but not responding to treatment) so I have 6 in my main tank, this is the absolute bare minimum group size so I will get more. Tank mates need to be peaceful. Mine have neon tetra, corydoras and red cherry shrimp.
A 25 gallon would be big enough for a school of dwarf rainbows but you may not be able to have many other tank mates. Larger rainbows need larger tanks. My shrimp are ok but they mostly stay in the plants. Rainbows need LOTS of plants as they can be skittish.
A website dedicated to Rainbowfishes of Australia and New Guinea
minimum size tanks for rainbowfish
2 foot tank for Pseudomugil species
30 inch to 3 foot tank for small Melanotaenia species
4 foot tank for all bigger species.
Keep the pH above 7.0 and GH above 150ppm for all species except wild caught Rhadinocentrus ornatus and Iriatherina werneri. These 2 species naturally occur in soft acid water but captive bred specimens will be fine in water with a neutral pH and GH above 100ppm.
Feed them plenty of plant matter.
Tank mates depends on the size of the fish you get but most barbs are fine as well as catfish and most bottom dwelling species.
25 is too small. I have kept Rainbows and Angels, my favorite fish, for years in a 30, 55 and now a 75 gallon. The 30 didn’t work. 55 was good, but limit your number of Rainbows to three/four. They are constant fast moving fish. I can see the difference in growth and general happiness with the length and depth of the 75. I have ten in there, Different types (I go for color!!) no problems. Rainbows grow to striking colors!! (Can’t tell when they are small.). Have fun!!