Hey Phillis,
There are only two companies that run ferrys to my knowledge and it doesn't make much difference to which one you go with. I either take the Hillarys ferry or the one that leaves from Rous Head. Same Same only different.
A bike will cost you $10 to go on the ferry and is worth it. Carry the kite on as hand luggage. I don't know of any kiters that live or work on the island regularly, although quite a few Perthians go over on boats at times.
Plenty of other threads on where to ride but Salmon Bay is the goods.
Have fun. Kiting is the only free thing on Rotto ..... for now!
Good winds,