Question: "Can I put cold wax medium over a hot wax piece. I have one or two pieces that I would love to cover but if I could leave the existing wax on the work I would prefer that, instead of having to melt it down. I am not even sure if I could cover it with acrylic or maybe even gesso first."
Answer: So, a "hot wax piece" refers to an encaustic painting. The good news is, YES, you can always paint with cwm/oils OVER an encaustic painting, but never in the reverse (ie, encaustic OVER cwm/oil)--why? Because heating an encaustic on top of cwm/oil is dangerous due to fumes and chance of fire due to solvents in the cwm/oils! So, you are SAFE to work on top of an encaustic painting with cold wax/oils; there is enough "like on like" for good adhesion.
With regard to acrylics, however, you can never put gesso or acrylic (both plastic in content) over an encaustic since there is very poor adhesion. IF you put encaustic gesso down FIRST on a panel, then you can work with encaustic, BUT, you can never put any kind of gesso or acrylic on top of an encaustic surface ;)