plecos usually do good in coldwater conditions, and since they are less active, they can live in a tank with less room than a different species of the same size can. lots of poop, though.
They may do fine below 70F but I would hesitate to go much lower than that...not many places would be able to house one outside long-term unless the pond was somehow heated.
In terms of space I would not put a potentially 18" fish (depending on the exact species) in a 75g unless you had plans to upgrade. Lazy or not they still need room to comfortably turn around.