Never kept them but I am thinking about it, was wondering what their growth rate is? How long before they start to get plump? And what is the best diet for growing them out?
They shouldn't be that slow up to 6" if given a good sized tank, warm water and good food. Not oscar-fast of course, but they should get to 6" from LFS-sized in a couple of years. There will be some variance from genetics and stunting depending on how long they've been cramped in tiny LFS tanks.
A good diet will help greatly, such as zucchini and algae wafers. If you take care of them properly then you can get their growth rate pretty high, sometimes they'll grow slightly faster than a rock.