Yes, you can park your domains for whatever time you see fit. Would it be couple of weeks, couple of years, couple of days or even couple of hours (yes, even couple of hours is possible - nowadays DNS propagation is almost instant). The only thing you will need to change when you'll feel like parking them at another spot is nameservers at the registrar.
This depends on your hosting company. Usually they make it clear to you, how many domains you'll be able to host/park/add with hostingplanA or hostingplanB. Sometimes it could be unlimited (although I strongly discourage you to using hosting which advertises 'unlimited' space and bandwith). Contact your hosting company if in doubt.
Good questions. Please let us know when/if you'll get an answers to them.