Are the SK636, SK638, and 636V2 universally compatible?

They are universally compatible. They can be used worldwide.

They require an AC adaptors (charger) with an input of between 110V and 240V, with frequency between 50Hz and 60Hz. 

A plug converter is required to convert the NEMA 1 plug to your local plug standard, or you can use a new AC adapter (charger).