Juveniles cannot be sexed. You would buy one adult male. So are you going back to raising one species at a time and selling off females? Or stocking adult males?
Look up these fish in the profiles and eliminate any that mature > six inches.
Choose one must have species and we can make suggestions around that.
Example for a more timid tank (my favorites).
Aulonocara Red Rubin Peacock 4 - 5 cm
Aulonocara Regal blue stuartgranti mbenji peacock 4 - 5 cm
Aulonocara Baenschi 4 - 5 cm
Not much in the way of haps <= six inches. If you go with fryeri, you may want more aggressive peacocks like these. Which jacobfriebergi? Lemon jake?
Aulonacara Orange Blotch Peacock, stunning colours 4 - 5 cm
Aulonocara Peacock Dragon blood The males are very red 4 - 5 cm
Aulonocara Jacobfriebergi 4 - 5 cm