Mt Douglas, Royal Oak, and Prospect Lake are not Par 3s. The first 4 courses are, and they are all 9-holes.
They have different pricing and what I listed there are average one. For example, if you go to Henderson between 4-5pm weekday then it's $4 or 5 (happy hour), Sunday morning it's $7 for a family (2 adults and 2 children). If you buy punch card then it's cheaper as well.
You don't need tee times to play in Par 3s. 2 other courses that don't need tee times are Mt Douglas and Cedar Hill Municipal which is one of THE BUSIEST course in Canada.
No need to get a foursome. Most of times I go out with my son only, and sometimes we join other groups.
Play 9 or 18? That depends on your time, money, how well you play the front 9, etc. Really a personal thing.