Based on temperament. But in a 30G you can only have one BP. Or max 2. Nothing else. If you have a 40 you can have them with Cichlids that are a bit hardy and can stand on their own. Convicts fair well I have seen. Usually goofy mouthed blood parrots are more commable. Having said that, I have a goofy mouthed blood parrot that police the tank and put all the fishes in their places including a 12” Oscar.
See what he does to a Flowerhorn female here and how a juvenile red-Devil runs for its life. So don’t underestimate a BPC.
I kept mine with in a 30 gallon with several fish for about half a year then I bought a 125 gallon to transfer them to. Good roomates would be something the same size or smaller and nothing aggressive. Try electrical blue acara, german or bolivian ram, any bottom feeders such as loaches
