What Track Brands are Compatable?
Most rail brands can be used together, though some are going to need a transition piece.
This transition might be as simple as a step in the roadbed to allow for the different tie thickness, making the top of the railhead even.
Sometimes it's the rail joiner that needs a step in it... this can be as simple as flattening one end of the joiner, and soldering the rail on top of the flattened section.
In other cases, the manufacturer may offer a transition piece from their fixed track system to flex track offered by another manufacturer.
To help in planning transitions, there is a Rail Data article posted here;