There are no plecs suitable for that sized tank.
The best thing for keep algae down on the glass is an algae scraper.
And on ornaments etc, I'd recommend one or two nerite snails or at the most an apple snail. However obviously this wouldn't be the smartest idea with puffers!
So in your case I would stick to ottos... Or perhaps try a couple of shrimp, I've known people have shrimp with puffers successfully.