His tank look pretty new, the fish don’t look to be adults, and he doesn’t have other fish in the tank with them. It also don’t say how big his tank is.
If that’s the harmoney you want in your tank why not just stick to one type of fish? And get all Bolivian rams?
Bolivian rams get a little bit bigger than German blues, if you don’t want breeding you could always get them all in the same sex to avoid extra