Can I completely replace the dirt sample w/ custom ones w/ the exception of a few that i want to keep for testing sound, or will that cause issues?
Use all, or none - it's up to you
The bigger questions:
why do you want to remove them?
If it's a RAM usage issue, then you may want to look here:
Superdirt lazy samples loading
how you plan to remove them -
So long as disk space is not an issue, I recommend doing it via the supercollider startup.scd - commenting out the default samples line.
If you just delete the files, you'll likely run into issues when you want to upgrade the superdirt quark down the track
actually both are an issue as i'm on an older computer, so might do the later i guess?
You should do both -
I should clarify, do step 1 first, because that requires that your superdirt dir is "pristine", once you've got it all set up on the developer branch and working with the lazy loading setting, then delete the superdirt samples in the folder