Shoe Goop would attack foam, but it will do a great job holding your servo to any semi pourous to non-pourous surface I can think of. If you are gluing to soft balsa, pin ***** the area, then hit it with CA glue. It's always a good idea to clean the servo case with something like acetone before gluing. This etches the plastic.
The Shoe Goop also isolates the servo from vibration and some pylon racers swear by it.
It isn't too hard to undo it if you have to later on, but it will put up a pretty good fight.
I used to use HS55s in all my 1/2A planes, but have stepped up to HS65s after quite a few HS55 failures. The HS-65s will still find ways to fail in the tail of a small 3D plane if you accidentally bump them or if the servos are forced to move the control surfaces [easily] beyond there normal range of motion.