I don't know what your cash situation is, but that Fender Rumble 100 sure would be a handy amp to have! You could use it for practicing at home, for home recording and all kinds of things. If you sell it, you might very well live to regret it. The last thing I ever feel like doing is setting up my rig to practice, to work on new material or to record. Total PITA. I've had a dedicated practice rig of some kind or another for eons now, and I can't see ever going back. A 12" loudspeaker is perfect, and that's what I'm using now. I guarantee you will play at home / practice / wood shed way more with a dedicated rig, and this will advance your playing skills significantly. [Note: Tip the amp back so it points at your head, or place it on a stool. Get a dedicated tuner, as well]. Just being able to leave your computer (or whatever you use for backing tracks) auxed in saves time, trouble and hassle. No more tripping over cable, or anything like that. No lugging you amp in and out of where you practice, setting it up, tearing it all down, etc. I hate that.
Fender's Rumble line is impressive. Treat it right, and it will treat you right. I don't know about the Rumble 500 breaking up. Any amp will break up, if you dime it out. I definitely would not get an extension speaker cab for a Rumble 500 until you have used it on stage at your gigs. It wouldn't surprise me one bit if you didn't need anything more. If you do get an extension cab, fine, but be sure to get the 2 X 10 so you have an array of the same kind of loudspeakers. Good luck!