Voltage rating for switch with high voltage but very low current
You should use a switch rated for at least the max voltage and current you expect to see (700VAC and 500VDC in this case). Using a relay is probably the way to go to isolate from the higher voltage. Having a higher current rating shouldn't matter, but here are some options that look like they meet your spec w/ relatively low current ratings:

http://www.digikey.com/product-detail/en/DAR70510/725-1081-ND/751985
http://www.digikey.com/product-detail/en/DAR70510S/725-1202-ND/2811118