Ah I see from your posts it appeared that you only had a 3ft tank silly me but you have a 6ft tank as well then you are in business
The best thing would be to get your 2 Clown loach some friends and keep them in the 6ft.
Keeping a single Clown Loach permanently in the 3ft on its own would not be ideal as they need there own kind, even if he or she is a small juvenile Clowns are very docile to my limited knowledge and have never experienced any hostility to any of my other fish mainly tetra's, Platy's, Cory's, Kuhli Loach and Rainbow fish.
So to answer your original question about Clowns being in with Guppy's and Platy's then that's fine as far as temperament goes, you would just need to make sure there requirements match i.e. water temperature, PH and environment by this I mean things like Clowns enjoy a good flow of water in there tank which imitates the rivers they come from originally but I think and do not quote me as I do not know much about Guppies but I think they prefer a more subdued water flow in there tanks.
Of course you can go on and on there is nothing like research and study of a species.
Its true they do grow large and bulky over time but they can live for a long long time. What I have read and experienced to a limited amount (been keeping Clowns for about 18 months) is that from the small 1 to 2 inch size they grow quite a lot for a couple of years then there growth slows down.
Here is a link to a good source of info on Clown Loach:
http/www.loaches.c...ia-macracanthus
I hope this helps you and apologies for the confusion, there are some far more experienced Clown Loach keepers on this forum that maybe able to support or correct my comments I hope they catch this thread
Regards onebto.