Best pricing strategy has always been to book as soon as possible, because as the dates get closer the prices just keep going up. I'm in the US though, so I don't know if that would apply to Canadians as well. You can always call Disney and ask if they have any specials for Canadians, or a travel agent. Sometimes the agents can get better deals at Disney World than booking straight through the website for us.
The rooms are pretty large, so fitting 3-4 people won't be an issue at all. On our cruise me, my mom, and my 11 year old son were in an interior room, like the cheapest and smallest one they offer, and were still fine. My sister has a family of 4 and were in an oceanview room and had PLENTY of space in there. They're pretty genius about their storage options, I was surprised.
No safety issues with the balcony, you can latch it. However, and this is totally a personal opinion, sometimes the balcony isn't worth it to some people. On Carnival I always spent the extra money on a balcony because we spent a LOT of time out there. On Disney there's so much to do that I didn't book a balcony room, and didn't regret it a bit. My sister sailed the first time in an ocean view room, and recently in a balcony and she said she'll book the ocean view again in a heart beat over a balcony because they just didn't use it much. So depending on how much time you'll actually be spending out there, it may be worth it to save the money and just do an oceanview.