Welcome!
Don't be surprised if you find yourself involved in Guinea Pig rescue before you know it, Cavies are rather addictive!
You are right that rabbits and Guinea pigs are not that different as far as care and eating habits go. It may be interesting the that whole idea to make cages out of grids and Coroplast was this sites owner, T's idea for guinea pigs, and has since been adopted by many other small animal people.
Hmm... Some of the differences between rabbits and pigs.
#1 is they need vitamin C, don't use the drops for the water, it degrades very quickly and is useless, also can be harmful because it changes the taste of the water and they may not want to drink it then. Lots of veggies, like sweet peppers (red are best) have vitamin C in them, See GL for more info. Alternately, you could get a good pellet like Kmshayloft or Oxbow which has stabilized vitamin C in it.
#2 They should not be fed rabbit pellets. Most pig pellets are garbage, KM and Oxbow are the only real good ones. If Rock and Roll are over a year, they definitely would benefit from a timothy hay based pellet, not alfalfa like most on the market. Also, if you feed a rabbit pellet with antibiotics in them, they can be harmful to the pigs.
#3.) Never leave a pig with a rabbit, while rabbits may want to play, they are much stronger and more then one pig has suffered extensive injuries from his / her rabbit "friend".
#4.) Rabbits can host, as I am sure you know, several different parasites and bacteria, such as Bortadella, (sp?) these can be fatal to a pig. Some people argue it is not even safe to house them in the same room let alone near eachother. I think it is just something you need to be aware of.
#5.) Guinea Pigs do not get "colds" and if they appear sick at anytime, it is a good idea to get to the vet asap. They can go downhill real fast, but otherwise, are very treatable with the right care.
Otherwise, I would suggest reading over the common misconceptions thread for lots of great care tips, and the Guinea Lynx care guide for raising a healthy guinea pig, found at
www.guinealynx.info
Most importantly, welcome to the world of pigs!