12V vs 18V vs 24V

I have the RB800 which specs say it requires 48V. Two questions about it:

If I use the PoE from a Cisco port, will that work well? I thought about removing all VLAN’s so it was just PoE and use the Gig port for the data path, hopefully it won’t cause a net loop.

Will the RB800’s run well on a 24V supply? I will only have one antenna off of it.