Im just about to add my new external filter to my tank i will still also run the internal juwel filter so my filter will be able to handle the overstocking with the cichlids. The question is as my tank is nearly at the end of the fishless cycle the addition of the new external shouldnt slow this process down should it as all the benificial bacteria will still be in the tank and internall. It will just be a couple of weeks till the external is fully cycled so my cycle will still be on course to finish next week. Does this sound right?
Adding another filter may actually speed up the cycling in your original filter as the ammonia load on that filter will be lowered.
Once the cycle is complete with the added ammonia just whack your fish in- adding another filter won't make any difference. With the cycle complete the internal filter will be able to take the load of the fish on its own (assuming your dose of ammonia is high enough, but again that is not filter related). Over time, bacteria will migrate to the external and bacteria will subside in the internal as a result. Whether you allow that process to occur with ammonia or fish is not relevant in my eyes- the tank will be able to handle the bioload at all times.
