For the longest time my 6 string fretted was my only bass I would use for slap (until I got a 5). and Ill tell you, it feels weird slapping on the 5 and not having the C string there.
You will have NO problem slapping on a Six... the only thing you need to do is make sure you are able to mute across all the strings with your left hand to keep things quiet.
My 6 is 34" / 18mm Ken Smith and its amazing.
A buddy of mine YEARS ago had a 5 string ken smith he converted to a 6 string... Crammed 6 strings on the 5 string, it was like 12mm spacing or something crazy like that, he had this technique where he would "thump" with a pick and pop with his fingers. bass was BLAZING for finger style, I couldnt slap on it as efficiently as other basses though.
I have some 6 string slap video's on my youtube.com/slapinfunk