Yup, brand mixing doesn't matter at all! I agree with the reptile basic's Ultratherm heat pads recommendation! Been super happy with mine!
I also second getting a thermostat over a rheostat! Way more accurate and reliable.
It's worth noting that the Jumpstart thermostat is a great choice as far as inexpensive options go, but best to be considered as a temporary placeholder for a better thermostat later. Or atleast keep in mind you may need to replace it later because cheap often means a shorter lifespan and less safety features. You'd ideally want one that has dimming or proportional heating opposed to on/off.
If you would prefer to get a long-term thermostat that will last you pretty much last forever consider investing in Herpstats by Spyder Robotics! By far the best of the best and worth every penny!
P.S - welcome to the forum! I've seen your artwork before and it's really cute!
Thank you very much for the recommendations.
Definitely considering getting a new thermostat to replace the rheostat at some point too. For now, I'm just glad to know its safe to mix brands, if it comes to that!