This may be a bit of a newbie question but...
If I use a source follower as the buffer in Sallen Key, will it work? Does the feedback scheme in the SK filter require gain?
It does require gain if you wish to use the equal components value implementation. I prefer the unity gain implementation as it keeps gain correct throughout the system.
the buffer is effectively a unity gain opamp.
This can replace a unity gain opamp in a unity gain S&K.
The Q and F are set by adjusting the component values.