Hello,
Yes you can mix them. The rhinoscriptsyntax python library is designed to emulate the syntax of rhinoscript. It is written in a functional way : you apply a function, you get a result. The result is generally updated directly in the Rhino document so you can see it working straight away.
It is written using rhinocommon in python. rhinocommon is written in an object oriented way : a point is an object with properties (x, y, z position,etc) and methods (functions, eg to calculate the distance to another object).
You can see the rhinocommon implementation of rhinoscriptsyntax by using this script : Python : May the source be with you!
You can mix both methods but you need to be careful as rhinoscriptsyntax generally refers to objects(points, etc) using their GUID number whereas rhinocommon refers to the rhinocommon object.
-Graham