Sponge Filter In Place Of Airstone

Yep - that would grow more bacteria and increase filtration capacity. As for increasing stocking... that depends on space and if you're prepared to up the water changes (if necessary) to cope with the extra Nitrate production etc etc. (But I'm sure you knew that... )

When I got the sponge filter for my betta's tank I went to the shop I trust the most. The chap said to get a sponge filter with a weighted base rather than the kind that stick to the glass with suckers. In your case it won't be the only filter so it isn't as critical, but the chap in the shop said he'd come across several cases where a sucker type had detached itself from the glass, floated and stopped working. If the sponge is only partly underwater it won't suck water through the whole thing.