I don't put my weight on my front leg when doing fs or bs slappys. all in the back leg, front leg just guides you into the initial truck bash and the back leg does all the work, you'll get a longer, more controlled slappy that way. I've been working on doing them switch a little more consistently and I notice alot of the time I'll have my weight on my front foot and have to do this ugly fakie pivot out of them, been working on fixing that. when you're first learning them the trick doesn't make sense so alot of people put their front foot on the nose or lean forward too much, it works but you can get pitched pretty bad if you slip out. work on leaning back and getting into them with your feet back a little more, if you miss the curb or slip out you can just kick the board out instead of slamming that way.
also I don't think small wheels really help with them or fuck them up, but really big wheels definitely make you lock in too much, anywhere from 53-56mm is the sweet spot for wheels imo.
gotta say spitfire classics are nice because of the rounded sidewall, you can roll right up some of the steeper curbs if you get your angle right. but your average parking block it isn't as important, just bash and lock on. curbs are fun.