KT88's are a superior tube to the 6550, with higher ratings and higher power output, and equal or better sound, but either one will work. For myself, and a lot of others here, the JJ KT88 is the best power tube for a 200S. Reliable, sounds good, good pwoer...what more do you want.....
There are a 4ohm, 8ohm, and 16 ohm tap in the amp already. The 16 ohm one is hooked to the negative feedback loop internally inside the amp head. You can change the wires on the back as far as which one goes to the jacks, and you can also change the extension speaker jack to a regular jack and run a tap to it since most cab jumpering is done these days at the cabinet instead of the amp. So you can have 4 and 8, 4 and 16, or 8 and 16 on the back of your amp by just changing a wire routing or two internally.
Greg