I'm not interested in bristlenoses, do I have any other options? Can't stay too small either as it'll have to live with 2 big polyupterus and an African knife.
I agree. Algae isn't a problem with proper maintenance and tank set-up. Algae is ubiquitous, but it requires more than just water to be successful to a point that it would be significant.
A persistent problem can be addressed substantially with a weak UV filter. (A strong one would be needed to kill parasites and most bacteria/viruses, but weak ones do yeoman work on algae.) Some people swear that the effect on water clarity of running one for just a few hours are startling.
Snails are definitely a good option. I like nerites and ramshorns. If you don't like snails My best algae eating fish is the Siamese Algae Eater. I don't see it bother other fish. They get around 5" but good luck finding one for sale that size. You will likely have to grow it out on your own. Depending on your poly that still might be a snack as it is still a pretty tube shaped fish.