From experience of finishing hand-built prototypes, I'd recommend removing as much excess epoxy as you can by hand with a sander/sandpaper before putting it through the machine, otherwise, you'll just gum up the belt with epoxy. although the main belt runs on a rubber wheel you will affect the geometry of the ski on the parts not covered with epoxy trying to remove the excess as the resin will be harder than the base plastic.
once you have all the excess off by hand, get a sense of how truly badly convex and/or concave the ski is using a true bar and giving them an initial (auto-feed) belt grind with a fairly worn belt.
If the grind is not reaching a large proportion of the skis and the inconsistency visible with the true bar is large i'd advise against just doing multiple passes until they're flat as you could grind away a lot of the base thickness in high spots, making them more vulnerable to core shot and generally reducing their working life.
I'd approach them accepting you probably wont get them completely flat and grind them with a combination of multiple low-med grind full-length passes and some open facing to grind the parts not getting hit with the full passes.
If they're particularly concave i'd avoid putting them over the stone altogether or with low pressure as you'll just channel the stone out with the edges to get a structure, either ruining the quality of other ski services afterward or creating a lot of facing off work on the stone to get it back to flat, which when a new stone costs thousands probably isn't worth it for a couple of pairs.
As for the edges, get as much off manually as you can and then grind them as normal, the belts are relatively cheap and sidewall material is soft compared to steel and so is going to grind away, removing inconsistencies in the edge arc/finishing