i'm sorry but by the time its too cold for a plant, you're gonna have a bettacicle.
i had a little 2.5 and never really had much of an algae issue. if you're really worried about it, your best bet would definitely be snails. you could get one ramshorn and/or one madagasgar trumpet with no worries of starting a colony. but once you get 2 snails of the same species, you're passing beyond a point of no return.