I don't know about the ones found in Ontario, which are likely non-native to the area, but here in Alabama, they only get like 5-6 inches maximum size. I would say a 75 could be a comfortable home to several, as they are less aggressive than most sunfish, and could even co-exist with things like koi or goldfish.
Orangespotted sunfish are one of the smaller species that reach at most around 6 inches in length. So yes, if the law there allows you keep several they would do well in a 75g aquarium permanently.
Most sunfish would do well in a 75g. Longears, pumpkinseeds, redspotteds, orangespotted, and dollar sunfish would all be good, colorful candidates. Nanfa.com forums has a lot of useful info and links to native fish vendors.