Hmm, I'm not too sure about moving the catfish to a 10 gallon. Is he fully grown? Might be ok for a while if he's still smallish, but in my opinion it wouldn't be enough room for him. But thats just me... If you do decide to move him though, you might want to steer clear of using a net to catch him. The pectoral fin can easily get tangled in the net and thats not great for stress levels. (for you -and- the fish!) Better off catching him with a glass or some kind of container/breeding trap or something...
As for the otos I'd be happy to put 4-5 in there as it is, but you might wanna keep it down to three. As long as you're not struggling to keep nitrates down every week, I'm sure they'll be happy in there. They don't get that big, and won't bother any of your other fish. One thing - Once they've quickly devoured all of your algae, (and that won't take long in a 20g) you'll need to feed them with wafers or veggies or similar. They seem to constantly graze on algae if it is present. Assuming you mean live plants they might snack on the softer variety if they get too hungry, but hopefully that won't be the case.
Oh yeah, acclimating them is a pretty crucial stage,its best to add very small amounts of water often to the bag while floating at the top rather than adding some and leaving for 5 mins etc. The first few weeks can be hit or miss, so monitor closely if you do decide to get some. Best of luck! =)