TBH 215L already sounds a bit small for 10cm clowns, if you want to keep them growing and healthy then need room to stretch their fins. What are the dimensions of the tank? I'd suggest rather than waiting until they get big in a small tank, upgrade ASAP, you'll see more steady growth and will be less likely to stunt them. At that size IMO they should have something 4' long at the very least, ideally 6' if you want to keep them on a roll.
27C is on the cooler end of the spectrum for CLs, but unless you've got plenty of aeration and good flow I'm not sure I'd increase it much more. That said, lying on their sides in a cave seems perfectly normal CL behaviour to me. Is there anything else that is making you concerned, cloudy eyes, losing slime coat, etc? Keep up the water changes and feed lightly until ammonia is back steady at zero.