On my trracer, that's basically how it works. But I'm sure some markers work differently.
Tube feed just means that you use 10, 15, or 20 round tubes instead of a hopper. Mine is a 10 round gravity feed, meaning I use 10 round tubes that I just plug into the feedneck. I need to get it changed to a horizontal feed... it just looks funny right now.
Stock class has a set of special rules, one of those being you can only use 12 gram CO2 powerlets.
www.stockclasspaintball.com and
www.ottersccustoms.com I think have the rules of stock class and modified stock class. I'd do either, but I'm going pump for the money, and filling a tank is much cheaper than buying 12 grams. I'm buying a quick changer, anyways, just in case I feel the need to go stock.
I've never seen SL-68, but I've heard it wasn't bad. I'm sure someone will give you a much better answer than that.
Pump is awesome. Saves money, requires more thought/strategy, and helps improve your game all around. I'm still not good enough to compete with semis, but I try. It feels so good when you get someone out shooting 5 or less shots.
PS: Rock the Pro/Carbine! My favorite gun of all time. I had to give it up, though.
I got an awesome deal on this Imagine, so it's all good.
I'm thinking about getting a p/c, just for old time's sake.