well idk if i failed to mention that i'm thinking of getting them as babies but yeah for sure you're right, after seeing that red devil, once they're passed like 6-7 inches the tank will be way too small all of them.Imo a 75 is only suitable for one 10"-12" fish. And you're talking about five fish that size.
And good luck only having two or three. Eventually you'll just have one owning the tank with the second and/or third fish stressed to hell.
when you say only one 10-12 inch fish, does that mean that only that and nothing else?