According to Audio Research the KT 120 is a drop-in replacement for their current amps. The Ref series amps are now shipping with the KT 120 tubes as standard. There were no other changes made to the Ref series amps. I know there are differences between the 6550 and the KT 120 tubes including that they draw about .3 amps more heater current per tube. This is not a problem in current production ARC amps. In the older amps there are concerns with the actual size of the tube and possible cooling problems. There is also a question of range on the bias pots of some older amps. My guess is that the KT 120 will work fine in a VT 100 but I would call ARC to verify.
The KT 120 tubes are available. They are just not available currently from ARC. This will change soon as they build up an inventory. When they are available they will cost approx $20 more per tube than the SED 6550.