If you are going to use a drifter, or a spinnaker, you will want another halyard to make it easier. On Serenity I had hank on jibs so I used the jib halyard for my drifter and a spinnaker halyard for the asymmetric.
Since you have roller furling you would need a drifter with a luff wire as you can't hank it on. You could drop the roller furling sail and use that halyard but you would be more likely to use the drifter more if you had a dedicated halyard. With a drifter you would want the halyard attached to the mast below the roller furler as you can tack with a drifter. Going dead downwind you could also try having the drifter on one side and the jib to the other.
If you were to go with a spinnaker the halyard should be above the roller furler so that you can gybe the spinnaker.
With either the drifter or spinnaker you would want the sheets run as far aft as possible. Tim's suggest is a good one, I put Harken 091 on the aft corners of the boat for the drifter.
On Serenity the drifter had a 17' long foot and almost came back as far as the main.
I loved the drifter on Serenity. On light wind days it was a lot of fun being the only boat in Narragansett Bay sailing. I sailed by a lot of boats that were becalmed. After sailing by them most would drop sails and motor away in disgust.
This was with the drifter out. 2.8 knots of wind and we were doing 2 knots of ground speed on a close reach.