BNs get to around 5, maybe 6". They'd be fine in a 20 long. Clowns are similar size. I don't know which would be better for a planted tank, though. Just returned my 6" common the other day because he'd decided to turn on my plants.
As far as algae control, im sure a bn would be better. My clown pleco hides in a crack in a piece of driftwood. 24 hours a day. the only time i ever see him is if i remove the driftwood. Neat looking fish but ill never get another.
You posted before i did lol. But from my experience a clown pleco wouldnt be a good choice. maybe mines just shy but thats my two cents
have a clown pleco in a heavily planted 29 gal with a school of dwarf rainbows for going on a year now. he has never messed with my plants at all. they stay small. he always hangs out on the driftwood. only time i see him is at night when he comes out for a green bean or algae wafer