Hi Don,
In my opinion, the super clear resin works better than the Easycast for heat resistance. While it will adhere to the EasyCast resin, it is not a doming resin. You will need to create a tape dam around your EasyCast piece before applying the super clear resin, otherwise it will roll off the surface.
I have always had good luck applying one brand of epoxy to another brand’s cured product.
Yes, the super clear resin can be colored and used in large projects, but you should mix no more than three ounces at once. This FAQ on the product may answer other questions you have about it as well: https://resinobsession.com/resin-frequently-asked-questions/resin-obsession-super-clear-resin-faq