Most synthetic strings, whackers included, are pretty stable once they stretch in. Whackers can take over a week to stretch enough to be stable.
I slap on steel and gut and have used whackers and synthetics in the past. You can mix and match as much as you like, but most people seem to prefer balanced tension across all four strings.
String height is a matter of string type and personal preference. Very low tension strings like whackers are comfortable to play 1/2" or more off the end of the board. Steel can be slapped comfortably with a jazz-friendly setup. You just have to have enough space between the board and the string to get the ball on the pad of your finger underneath the string -- there is no good reason to hook the entire first knuckle of your fingers under the string.
Blast Cult and other slap-friendly brands set up their basses with flat fingerboards a la Engelhardt. I have no problem slapping on a bass set up with an orchestra curve in the bridge crown.
The technique is very similar to dribbling a basketball.
"All of the poor people who started rock and roll are cool." -- Iggy