While I don't think could add a new species of fish into the 29gal but you should add more cories and could do so without overstocking - but there isn't really room for another shoal of fish or a pair of something (ie kribs or rams).
I really wouldn't advise putting a dwarf gourami in a 6 gallon... A dwarf gourami will still get to be over an inch long, almost 2 inches, and an inch high. I have 3 in my 55gal and they use much of the swimming area available. You could do it and get away with it but I really think it's too small for a dwarf gourami.
The 10gal however could look great with a gourami
if you really do want one.