The CF moves approximately 15,000 members annually and the vast majority of those moves occur during the summer, so yes, that's a busy time.
PMQs are normally assigned by family size, but you can ask and if a larger one is available, you can normally get it. You will have to pay the higher rent though. If you are initially assigned a smaller one, you can ask for a larger one as your family grows, but you may be on the hook for the moving costs to to the new house down the street. If you can somehow time your family growth with a major rat infestation, your moving costs (a "local" move) will be covered, but don't count on this as a course of action.
Yes you can decorate to a certain degree, but keep in mind that you may be required to return the PMQ to its original state when you leave (this can cost money if professional services are required). Changing the light fixtures may be possible, but you will need permission from CFHA and they may (in fact, they likely will) insist that they be installed by a licensed electrician and you will likely have to pay for that. The key is to ask. If you're thinking that no one would ever know if you just changed them yourself, you're likely right, but are you willing to take that risk? The CF takes a pretty dim view of people burning down its houses, unless your kids are playing with matches (I'm not kidding, that's actually in the book). Your tenancy insurance (which is required) won't cover you if the house burns down as a result of you playing with the wiring.
Good luck. Moving is such fun!
