Why not use sand for the bettas and rocks for the goldie? :) You can get some lovely smooth, polished, tumbled river rocks like this: http://rocktumblingsupplies.com/imag...xed-medium.jpg
that would fit well in a ten gallon (I've seen them in use) and provide a surface that is smooth enough for your Kia, yet too big for Beau to swallow. Laid flat, they don't pose too much of a danger.
If you have facebook, search Acumen Aquatics. The lady who runs it does some gorgeous tanks with river rocks that might inspire you. :) She keeps cories in them.
In a cycled tank, draining it is ok as long as you keep the filter submerged in the dirty water you remove (if possible, keep it running in there). The filter holds 90% of the good bacteria, so at worst you'll have a tiny mini-cycle when you put it back it.