I'm coldworking drop ring vessels using a Waterjet Design multitool to cut the tops off. Their video shows someone using a diamond cone to grind a bevel on the cut top. Would you recommend a medium grit and if so which threading should I choose? Thanks.

You'll want a 5/8-11 threading on the cone for use on that right-angle grinder.

The medium cone will work the fastest for removal, but if your rims are thinner, you'll want to use the fine grit to prevent chipping. 

Typically, with drop vessels, the rim will be thick enough for the medium-grit cone to work best.