well looked after Comets can get over a foot long, I know they are probably small now but they grow surprisingly large.
I've kept comets before and they've gotten up to about 8 inches. Surprised me, mostly because I was 10 when I got the fish, because of the next few years it just kept getting bigger. I was under the notion that it was just a feeder and wouldn't get too much bigger. I'm 23 now and would probably still have that fish if my mom didn't make me break down my tank.well looked after Comets can get over a foot long, I know they are probably small now but they grow surprisingly large.
The 10 gallon is just to see how my nephew does with the tank. He is 4 after all. I have a 120 that I am going to eventually set up for him.In this size tank the only suitable algae eater would be snails/shrimp and maybe otocinclus. I would not add shrimps or otos though. Nerite snails are probably your best bet.
In case you don't already know you'll want a 75g+ (preferably a pond) for them. The fancies I believe you want 20g for the first and 10g for each additional. Not trying to bash you or anything.