I have a few rc's with pull starts but was looking into a starter. was woundering if I can bump start with the pullstarts or do I have to get different backplates for engines.

You can start most if not all RC Cars with a starter box as long as the chassis has a hole to access the flywheel, but remember that not all cars have the same engine configuration. Some cars have the engine mounted straight and others have the engine sideways. Unless all your cars have the same engine configuration you will not be able to use the same starter box for all of them. You can, but you would have to change the wheel on the starter box and that would be a pain.

If you want to convert a pull start engine to start with a starter box you will want to find out if the engine manufacturer makes a solid back plate for that specific engine. That way you can get rid of the pull start and use the starter box.