Appearance is where the similarities end with those ports. They are wired completely differently. You probably wouldn't break anything, but it also wouldn't work, if you managed to get that mouse plugged in.

If you have one of the G3 PowerMacs that was pre-USB based mice and keyboards, then you either have to get an adapter or a new mouse. eBay is an excellent place to track such a thing down I might add. You could also check with any local schools or universities. They might have a few stuffed away in a drawer or closet somewhere they'd let you have. At my last job, there was a whole, rather large, filing cabinet devoted to just storing odds and ends for computers. One whole drawer was devoted to spare keyboards and mice. Some larger schools will have regular sales where they allow the public to buy old equipment they wish to dispose of. There are plenty of options if you know where to look.