Main speakers tend to be bigger and bulkier than center channels too. The center is most always built to fit on a shelf or something like that. I don't know how you're planning fit to main as a standalone center. The center channels are often matched to fit the response of the mains. And in many cases the mid and highs can be exactly the same as the mains. It just may not extend as much as the mains can do that for it and/or that extension would have been re-routed to the sub anyway. This is part of how the center tends to be much smaller than the mains.

IMO, if the speaker line you are looking at is of any quality worth getting then they can make a center that will match in with the mains just fine.