I think you should wait before using it and buy a better psu, as the 2070 super has a 215 tdp which will also be + cpu ram (not alot but still) and drives (again not alot). But to your question it is safe just don't downclock too much, you'll have to test which clock will be the most stable.
I don't know how underclocking could harm it at all... it should help it, actually, as it reduces electrical and thermal stress on all the components. It will just be slower.Thanks I'll wait it out, not trying to destroy my gear.