I wasn't aware of any tires coming in a 7 inch size but if that's the case, then I'd contact some of the sellers on e-Bay, such as Rustybore or Miller tire. Those guys are the Pro's and can tell you quickly if that is a correct tire size or not. They can also tell you what tread patterns are available. That will help you decide whether switching to an 8" rim size is necessary.

All that said though, if these wheels are not powered by the engine, then tread pattern isn't relevant.