Just let them as their own component, no need to group them together or anything. C2 cars can have a myriad of components.
With PT2, you must first import the ACT file, it'll then ask for the DAT and MAT files, PT2 will look for the tiff files into the tiffrgb folder. When you save the modifications, it'll create an SDF file that you can open directly the next time. Keep in mind that modifications you make in PT2 can't be loaded by CarEd, so PT2 must be used once you're done working on the model within CarEd (or for quick ingame tests ofcourse).
I believe C2 has that 1024 vertices limit because of the dynamic deformation applied to all crushable elements (by default everything but the wheels). C1 has precalculated crush datas for only 3 models (main, bonnet and simple models) so it didn't force the vertice limit. C2 however seems to prepare each car element for crushing upon loading the race, I imagine having too many complex models to deform could have been a strain on computers from the late 90s so that limit was put in place.
It's easy to work around it though: either split the models in several elements, or put a groove on the hipoly element to disable crushing (not sure it can be done on the main component though).
Don't forget you can check the vertice and triangle count of each element from within PT2 via the registry (Edit > Show PT2 Registry). And also optimise all elements by selecting the master element, then Object > Misc Obj Operations > Optimize. Can also check for errors with the same path.
BTW, it'd be better to create a new topic in the C2 subforum if you start releasing and discussing C2 stuff