These little guys eat like crazy... but just are not growing much at all.

That's pretty normal for most Loaches though I believe isn't it? I know my clown loaches are that way, very slow growing. I've had my Loaches for about (6-8 months maybe?) and I really cant say I have noticed much (maybe 1/8-1/4" at most) growth in them so far even though I feed them algae wafers 3-4 times per week.