You could get tiny cories... these guys are fully grown :) They are Corydoras Habrosus and look/act more like a "big" cory than the pygmy cory (corydoras pygmaeus) does. I think they are my new favorite fish because they are SO cute and so incredibly active.
(not the best pic, I took this with my phone)
My betta doesn't bother them at all.. he was going after the sinking food the cories were munching on. I wouldn't hestitate to keep a dozen of these along with a betta in the 6.6 - they are really quite small.
Just make sure you dont buy from them... we have had 3 failed consecutive orders in the past few months by our local community. they dont bag properly and plants and livestock came dead or dying. an entire bag of panda corys came fried being placed next to a bare heat pack. just giving fair warning...
Yeah, don't order from them.. they've sent out mushy plants, dead livestock, and livestock that seem to die off in the month after being shipped....
And this is after trying them a few times to see if the DOAs were just a fluke incident. it wasn't.
To their credit, they did send replacements but the whole order process was very long and drawn out with documenting DOA's with photos and multiple correspondence after the fact not to mention the several weeks in between the initial order and when the replacements finally arrived.