Will it fit? O-54 Miminum Curve rated locomotives & rolling stock on O-48

My guess is that most would be capable of negotiating O48 curves without derailing, but it would be a gamble. Some may not. Also, consider that sharper curves might be possible, but long hours doing so could cause damaging wear.

Kyle, since you have only 12 x 6 to work with, IMO you can build a much more "fun" and varied layout with small-radius curves.  Personally I would ditch the O48 in favor of all O36.  If you step up to O54  you'll be limited to a stubby oval or wide circle that doesn't offer much operational variety.  So my advice is to leave those big locos in the store and think about all the possibilities your space offers with O36 or even O31 curves.  Locos are available with great features and smooth operation on those curves, including some small scale models.  My $.02.