My question is does the AFF I rating override the coach rating or should I get this rating also? Is it needed if one is an AFF I?
If you are an AFF-I you are able to do anything a coach can do and more; no reason to get a coach rating if you are going to do an AFF-I immediately after. If you were wanting to practice working with students for a while as a coach before going for the AFF-I, that would be an option, but in your case it seems like straight to AFF-I would be what you are looking for since you meet all of the requirements.
In reply to:
Also, what can an AFF I sign off on as I live outside of the USA but I want my club to be affiliated to tidy up our ability to get a licence for new members.
This would be great material to start reading up on before you go to the course, all of it is found in the SIM and the IRM.
The question is "do you currently hold any USPA instructional rating or any instructional rating from any recognised parachuting organization"? Having a tandem manufacturer rating only doesn't count. If the answer is no, then you would have to get the coach rating first. If you have non-USPA instructional rating, there is a procedure for converting them. Get an IRM or contact Jim Crouch at USPA to get the facts.