I would advise against the exact scenario you suggest.
Williams units start in forward. Some other brands start in neutral before forward.
The biggest problem is that the motors in the K-Line units may not (probably will not) match the Williams speed/gear-wise and will fight each other. This is particularly so if the K-Line unit has truck mounted motors. Can motors are not as forgiving as pullmore motors. The faster running can motors will try to do all the work and will get hot, dragging the slower motors along.
The K-Line shell is not a direct fit to the Williams chassis.
The K-Line locomotives are not as tall as the Williams F7 and will not match well aesthetically. Are you sure the Williams is an F7 and not an F3? Big difference in Williams terminology/design.
My suggestion is to find any Williams F7 powered unit. The motors/trucks will bolt right in and are the same as Williams:
-GP9
-BL2
-The BLUE Chuggington engine (although the reverse unit is different)
-copy of the Lionel postwar 44 tonner (not the scale version)
-Alco FA-1
-most Williams two axle diesels except the F3.
Also, you should be aware of the vintage of your existing Williams F7. They had different motors and trucks over the years. The earliest ones have an aluminum chassis, pickups close to the belly tank, and no flywheel motors. You want all of your powered units to be of similar vintage, similar motors, and same brand (Williams).