Putting aside that 10-20 gal is a bit too small for Angels, for algae eating you'd be best off with otos or SAEs. I have a 72 gal with two BN plecos, 3 SAEs and about 6 otos. The otos and SAEs are by far the best algae eaters in the bunch. The plecos are busy but I do not see them going after the BBA, GSA or hair algae that arises from time to time like the others seem to. The BN plecos are really cool fish though (but also a 10 gal would be too small for anything more than 1 BN pleco anyway)
That said I had a nice pair of marble angels in this tank and about 2 weeks after I first got them one of them ate an oto. I should say got one stuck in his/her throat. About 2 weeks later it made its way down. Likely was an uncomfortable experience as neither angel ever ate another fish be it oto or any of my tertas and it's been almost a year now.
In the end though while some algae-eaters can be nice do not get them with the expectation that they will cure all algae whoas. Good tank maintenance and proper plant, lighting and fert regimes will ensure algae stays under control. As no amount of algae eating fish will cure poor practices that result in a forest of algae issues.
But really, if you're talking a "tank of angels" (as in plural) you really need a bigger tank than 10-20 gals.
