How to extend a specific node type form?
AFAIK forms are tied to operations at entity-type level, so you don't have any hook to only alter the form for a particular bundle. Since what you want is to inject certain services for a given bundle what you can do is to implement a chain of responsability pattern through service collectors. This allows you to inject only the service collector to your form and delegate to the collection to evaluate which service should apply for your given bundle. And finally the resolved service is the one that would concentrate the responsability. I could provide you with a deep example but could take some time, let me know if a code sample still needed.

UPDATE

This will be the solution in a near future, since sub-classing is the feature you are referring.