Not sure about new strings. I’ve got a Squire Jazz from about 9 years ago and it still has original strings. It hasn’t been played a massive amount but it’s been left out a bit. It still slaps ok.
Don’t worry about strings, with older strings you just wont have that zing, which you can partially recover by lossening them up, and poping them hard against fretboard (pop them on every part of their length). So what it does is gets rid of gunk between wounding. Then you just wipe the strings, you will hear the difference.
As I probably said before, beware of action cause high action is what makes slaping hard.
Lower the action, less power you need for slap.
If muscle memory/technique is the issue, dont worry, it will come with time, just keep doing it.
I have watched every tutorial video on the internet and I still can’t manage to get any decent tone from my slap. Obviously I am the problem here, for whatever reason I just can’t seem to make it sound even passable. There’s plenty to play that doesn’t involve slap but I love the tone so much, I wish I could figure it out. Considering paying someone to teach me at some point so for now I have given up on slap
Slapping is a bit different because in general you’d want a snappy clean tone so round wound would be the best choice, best stainless or coated string I’d revise the Ernie Ball Slinky or super slinky as well as Fender 45.
Non slap applications is different some want them old and grimy for a nice warm tone, ABBA bassist is known to “loan” his new basses out to a friends for a few years to “break in” the strings. I wish I’m not germaphobe OCD, lol.
I upgrade the pickup and pickguard. On the first one you see, I swapped out the neck to the roasted maple neck, install pickups and electronics as well as the Hipshot Tuner, bridge, and pickguard, well yeah pretty much everything just started with Fender Body and Neck and upgrade everything else, lol.
Next project I have a black body, red EMG pickups, black Babicz bridge, not sure what tuners yet. It would be the BlackPink Theme like the singing group, lol.
Black or pink string you’d give up on the clarity. Color coated strings tends to be a bit duller which is ok for anything else but slapping, knit picking of course. It sure would look good.