I take 300mg of Lamictal/lamotrigine. I get Botox. Can I have dermal fillers in my cheeks? Or will it interact?

There should be no interaction between the Botox , medications and filler injections. I would not worry about getting filler injections. 

There was a recent report that looked at 46 people on Lamictal who got botox. The main drug interactions included nausea, blurred vision, vomiting, dizziness and trouble swallowing. 
If you have been getting Botox and haven't experienced any of these then that is fantastic. Just be aware that there is that potential. 
As for dermal fillers, I would advise you to stick with dermal fillers that have hyaluronic acid in them as they can be removed if any reaction occurs. The bigger issue would be with the lidocaine that is in most fillers. 
That said, there is nothing in the literature that suggests that you cannot have dermal fillers.
Get your cheeks augmented and put on a big smile as there is nothing better for self esteem than liking how you look!