Another thing to consider is the overall relationship of your equipment's size to one another. As others have mentioned, the nomenclature can be confusing - Railking, Rugged Rails, RK Scale, Imperial, and the like. Premiere generally conotates 'true 1:48 scale' equipment. The Railking line generally incorporates various degrees of 'compression' or size reduction. If most of what you will have will be of the smaller sized equipment then the true scale (whether RK Scale or Premiere) can look slightly oversized.
If this is not a major concern (as it is with many of us) mixing them is fine. There are many Premiere engines and rolling stock items that can handle the tight curves and blend in well with the non-scale equipment.
Most of all its YOUR RR so have fun!
