Hello everyone- I've had a small Syngonium for a couple of weeks under a grow light and read that they like humid conditions. I had already potted it up with potting soil and a good amount of perlite, so I got a fish bowl and put the potted plant inside for a little terrarium. I then heard that when making terrariums you should use cactus mix so it doesn't stay too wet..should I repot the plant in cactus mix? Or is it alright the way it is?

Should be fine.
Make a false bottom, with spaghmum or gravel and such as to control moisture.I'd say one inch of false bottom should be aquedate.

If the terrarium is closed (with a cap or something), I suggest you take it off daily or every other day to wipe off the "sweat" or condensation to make it easier to see and to get the excess moisture out.

You might want to consider a smaller groundcover to go with it, such as oak leaf creeping fig (My personal favorite one to use!!)

Alright will do! Thank you!