Welcome to GBO!
As pills said, they won't sell barrels without fitting them to your frame, each barrel is fitted individually, they aren't one size fits all. Having more than one frame helps insure that buying a used barrel will minimize fitting issues, but doesn't necessarily mean that a used barrel will fit. I have twenty SB2 frames, i bought a used 7mm-08 barrel from another member here, it would only fit one frame without any work, all the others would have required some modification to the pivot for a good fit.
The rimfire(SS1)/shotgun(SB1) frames are cast iron frames, even if the firing pin wasn't an issue on the Sportsters, they aren't strong enough for most centerfire calibers, centerfire frames are SB2, made of better material and heat treated to stand centerfire pressures well in excess of rimfire and shotgun pressures.
The combos haven't been available yet with the exception of the .22/.410 which has an offset bore on the .22Lr barrel, we guess that the 3 barrel combos will have offset bores on the rimfire barrels and the frame will be an SB2 frame.
Read the FAQ and Help, there's a lot of info in there that will answer your questions, including H&R's email address.
Tim