I ended up getting 3 harlequin rasboras and 3 platys,
I will say that something I blanked on is that you've got a small tank and zebra danios are very active swimmers that need room. I think what you've got now is good.
and am babysitting a friend's oto while he's away on vacation (the oto is in my tank until he gets back). I think once he gets back and takes his oto back, I'll just get one maybe. I want something that will help keep the tank clean, and I think otos are cute and fun to watch... but if there won't be enough algae for 3 then maybe I'll stick with 1.
You already have something that will keep the tank clean. You. I'm not being mean but it is your responsibility to keep the tank in the condition you want it, not the fish's. I have never heard of anyone with a freshwater tank successfully combat algae with an animal they introduced to the tank, ever. There are too many factors at play to ever be successful in this way. If you think the otos are cute and fun to watch then by all means get 3, they like crowds. Suppliment their diet with sinking algae wafers and they'll likely be fine.
Hahah probably not (at least not in the way we understand the concept) but who knows if there's some other reasoning behind it than just plain math?
I have never seen any actual reasoning behind that bit of advice, not even bad reasoning. The explanation I usually hear is, "Well that's what I was told." Most shoaling fish don't exist in small groups in the wild but in groups of hundreds, even thousands of fish at a time. There's no way a human can count them much less a fish. When it comes to fish like tetras just know they like lots of company, the more the better. You'll get better activity out of them and they just look great in large groups. Other fish, like tiger barbs and cichlids, need to be kept in large groups to spread around the agression and bullying they can get into so that the weaker fish aren't systematically killed off.
I had read a suggestion elsewhere that it's more about keeping the 2:1 ratio between females and males with livebearers, which does make a bit more sense.
That is actually very good advice and I would consider it an absolute minimum as well. Like I said earlier livebearers have adopted the survival mechanism of just making so many little fish that some are bound to survives. Consequently male live bearers are horny little devils and will pester the female incessantly. Just like with the more agressive fish you keep multiple females in the tank to spread the male's advances around.
I believe I have 2 females and 1 male platy.
Well then mazal tov, both your females are pregnant.
Yeah... just sometimes I feel like I've made this near-instant transformation from someone who only thought about fish as food, into someone who can't stop thinking about fish and how to keep them alive and happy haha.
That's just sort of the way it works honestly. Though I still enjoy a nice piece of sushi.