G'day Kevin,
good choice in getting the juvies and watching them grow. In the breeding tank I have the larger size CFM sand and this works a treat, they sift through it, keeping it clean although it can get spread over the breeding rock. I also have a large female in a six foot and juvies in a separate tank both with that brown gravel mixed with a small quantity of smooth black gravel ( I like the look of this). I've found the juvies colour up sooner with this darker substrate and the large female has exceptional colour over this also, although she did spend summer in my pond after removing her from the pair(trio). Her colour and health was amazing coming out of the pond.
I set up some rockwork and placed fake plants ( they even fight with these), and this seems to give ample chillout space when she needs time away.
Cannot help with clown loaches, they've always scared me
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But as you say they grow large and should give an intersting contrast to GT/RT.
Sailfins Gibbiceps are a great fish that suit the American setup your thinking of.
If your planning both GT and RT, you'll obviously have to limit/lessen your numbers as they grow, but I'm sure your on top of that. Maybe just ending with a couple of fish, no pairs, and a couple of cats.
Filtration sounds perfect, I am an Eheim fan myself, never had a problem( except one Jager heater but thats another story)
Just to throw a spanner in the works have you thought of an Eartheater tank.......
cheers