I have experimented with swim feeders and i myself cannot yet say wether or not they have increased my catch rate, i have decided to only use one when fish are not biting to see if the extra scent trail helps. I have also thought that possibly the fish may have a go at the feeder as a few times they have come back in looking like something had had ago at it.
Anyway back to your question, i myself put the feeder at the bottom of my rig just above the weight, i find it cast easier, but i cannot see why you could not put it where you like on rig.
I've used sea feeders with luck for flatties in not too streamy water or from pier/close range fishing. In the most obvious case I caught most of the fish even though my fishing mates stood ten feet to the sides of me. Mashed boiled shrimps with fish oil and mashed common mussels has given best effect. Also check the surf feeders at Sharnbrook tackle, use pulp of the same as your bait. The scent trail seems to attract especially cod and flatties. Must be said I fish in Norway, but I guess my experiences can be transferred to British conditions.