Hey BF mechanics, I have come across a set of Shimano RSX levers, and would like to install them on my '88 Schwinn Prelude. Currently the bike has Suntour Accushift components. The freewheel on the back is a 6 speed. So, what i gather i will need to do is acquire a rear derailleur that is compatible with the RSX levers, and change my freewheel to a 7 speed cassette. I would then need to change the length of my chain to accomodate the larger cassette. Now here is what I don't know- What derailleur would be compatible with these RSX levers? Would a Shimano Tiagra derailleur work? I have installed the FD lever, and it seems to work fine with the Suntour Accushift FD. Second, how will I know how much I should lengthen my chain by?

I am slowly learning how to work on my bike myself, so I'm looking forward to learning about this. Thanks!