Imo I agree. Clown loaches are cute when small, get pushier with size, capable of getting aggressive in some cases. But I have seen odd combinations work, so not being dogmatic. Could be one of those sometimes it works, sometimes it doesn't situations.
I wouldn't call CLs slow growing, ime they grow pretty quickly to @ 6,7 inches, then, like a lot of fish, grow slower to their eventual max. Cool fish in their own right. Imo the keep them until they get too big theory might be okay, but then you have the challenge of finding a home for larger fish.
Imo, with your stock list it comes down to this-- do I want a GT tank with fish to complement the GTs or a clown loach tank with some GTs in it, because that's what 6,7 loaches and a few GTs would become imo-- unless one or the other decided not to tolerate each other.