If you want a quality HOB filter, get an AquaClear. My rule of thumb for any tank is to use a HOB that's rated for twice the tank size. So, for a 15g, I would use an AC30.
I understand you're in Canada, but are you sure you have the wrong type CO2 cylinder, rather than the wrong type fitting for your regulator? Regulator nipples and nuts are generally easy to change.
ADA products can be bought from the ADA website.
CEC, for our purposes, is simply the ability of your substrate to absorb nutrients. Flourite and EcoComplete have high CEC's, while being inert (that is, they add no nutrients to the tank on their own). I've heard great things about ADA substrate (once people get past the ammonia spike stage), but remember that any substrate that comes stocked with nutrients *will eventually become inert*. Yes, it'll take a couple years, but it's something to remember.