I would at least go with the 4.5" if you want a shad imitator and then 6" if you want a trout imitator. I wouldn't go with anything small than that. You should probably be able to use the 4.5" shad, but I don't know if you have the gear for the 6" trout. I can probably BARELY use the weedless 6" trout, but I'm really pushing it with only 12 lb test on my baitcaster.

how many yards of 12# does your reel hold

160 yards. Would I be able to hold enough of a heavier line that I would use for swimbaits?

The world record bass couldn't pull off 50 yards of line without having a heart attack. They are not built for that. You could go to 17# or 20# and still have enough line to fight a big largemouth. You will sacrifice casting distance though as the spool diameter will decrease rather quickly as the line goes out increasing drag on the cast. Go with a wide spool reel and you can go up in line weight and still cast sufficiently. Pick up a used ABU 6500c and up grade the bearings, you'll do fine.

I was thinking about getting another rod and reel for not too much money for bigger swimbaits. I wouldn't even attempt to upgrade the bearings though...

Would this be an ok choice? I can get it pretty cheap at Walmart.
http://www.basspro.com/webapp/wcs/stores...0001000_100-1-1

That particular model ABU has some serious issues. Look for a used 6500c3 on Craigslist or ebay, or go to www.ffo-tackle.com and get a reconditioned one. Even a 5600c wide spool would work. I buy lots of used reels and find out that most just need a good cleaning. I'm not big on cosmetics, if a reel has some wear, so what.