Yes, of course it is. You can use as many instances of any given class as you want. If any of the VIs in that class store data inside the VI using an uninitialized shift register, then you'll need to make those VIs reentrant.
For a plug-in architecture, a common system is to load the class from disk when needed using Get LV Class Default Value VI. You can call this as many times as you want to get multiple instances. You'll then need to cast them, using To More Specific Class, to a parent class that is statically linked to the VI. While the wire will be the parent class type, at run-time it will call the child class' methods (the class that you loaded from disk).
