Would a group of otocinclus work in my tank?

I think 4-5 otos would do well in your tank. If you are dilligent in keeping your water quality, I'm not too concerned. The other species don't seem to be a problem either, and won't pick on the otos. Otos do like a planted setup in my experience, and don't spend much time on the substrate. And snce they don't have fragile barbels like corydoras, there's nothing to be eroded away. Only concern would be their bellied. I would look up the species of algae your friend has, algae eaters are very particular and the otos might not eat the type she has. I say give it a shot. If not, use algae wafers, cucumber/zucchini, those seaweed sheets, and lettuce. Also make sure the betta won't pick on them, but since they're not colorful or flashy I believe they'll be fine.