My Clowns grew to 7 & 8 inches in the first year I had them (pics in my album). so I know personally they get bigger than 6 inches in home aquaria.
You may have read this article already but if not it's interesting and there are some cool pictures in it.
http://www.loaches.com/species-index...a-macracanthus .This particular site is right on with their loach information. The articles are written by people who own the loaches their writing about.
Clown Loaches need lots of room to swim, warm highly oxygenated, extremely clean water.
My Clowns are more active in the early morning and late evening, displaying the exact behaviour you describe. It's frequently described as "the loachy dance". corys will shoot from the bottom to the top for a gulp of air also.
I see that your tank has only been set up about a month, carefully watching water parameters (
ammonia, nitrites and Nitrates) and keeping the water temps at/above 82f will make your loaches happy and prevent ICH. Clown Loaches are like ICH magnets but the warmer temps will prevent that.
Keep us posted as to how things go

Carol