Some people use tuna rings to attach floro to mono but I use the Seagar knot (less number of knots to tie). Its easy to tie and I have not had one fail. I use about a 6' piece of floro some people like it shorter and some longer make it so when you are casting the knot or ring does not go into the eyes of your rod. I am a big fan of ringed circle hooks for tuna and tails and use J hooks for other fishes. The hook size will depend on the size of the bait and each maker has its own sizing system but from about 2 thru 2/0 should be fine. Some people use the same pound test as their mono but I bump it up one line class say 20lb mono I would use 30lb floro.
When we were in Louisiana I looked at how the charter captains had their Fluorocarbon attached, most were using small Spro Power Swivels and tying on 10' of Fluorocarbon. Smaller hooks, I dropped down to 3/0 Gorillas, and Fluorocarbon made for more hits on YFT.
