Basicly from the modding perspective there is only 2 classes frigates and capitals. The difference is the way their file is built and other limits like only capitals can get experience and frigates can be deployed by abilities while capitals can't. Also Only frigate type files/entities can be build in the factory, while only capital files/entities can be build in the capital shipyard. You can implement more classes by giving ship different research requirements so that there are some that you start with and then there are those who require more resources and higher level of research.

I hope that answers your question.