Anyone have any concerns of using wax coated ice cream containers for mixing up epoxy?

Yes. Wax dissolves into the epoxy and can cause issues later. That said, surfboard builders will often ADD a small bit of wax into their last "hotcoat" of epoxy to make it flow better and ease sanding. Not good for laminating as it causes bonding issues.

For mixing containers I buy sleeves of cheap, clear plastic, disposable cups from Wally-world. I use the 13oz for most layups. The 9oz ones are good for small batches. A sleeve of 50 is like a buck.