Clown loaches should be housed in a 6ft minimum tank (doesn't matter if you have 1 or 5) they are very active swimmers. They generally grow fast for the first year you have them, upto about 6" then their growth will slow down. They can grow upto 16" in the wild and in the aquarium when they have good living conditions.
Here is a good article about them with pictures of how big they can get to:
http://www.loaches.com/species-index...a-macracanthus
If you say yours have not grown much the chances are they are already being stunted from having too small a tank! Their internal organs will continue to grow but the overall size of them will not, this causes suffering of these fish before they eventually die (this is the same for any fish kept in a tank too small for their needs).
I would advise to return the clown loaches to the
LFS or get a bigger tank ASAP.
On the note about the RTS, i keep a RTS with my CLown loaches and he will chase them as they have a similar body shape and a red tail (RTS are more territorial with fish that look like them) if you have more than 1 clown loach the aggression will be shared amongst them. My RTS has learnt that the clowns are no threat and will now school with them and share caves with them.
Anna
EDIT: if they have ick they are already showing signs of stress from being in a tank too small for them. I would increase the water temperature to 83F for 2 weeks and add a air stone to increase the amount of oxygen in the water (increased temperature will speed up the circle of life for the ick so it dies faster, and the air stone is because warmer water holds less oxygen). Make small daily water changes of about 10% whilst vacuuming the substrate. Vacuuming the substrate will remove the cysts that fall of the fish and prevent the circle of the life cycle of ich. No need to stress them further with putting them into a quarantine tank, this method i said will treat the whole tank and help prevent the other fish from getting ick as well.
Good luck with them