RB951G-2HnD can use UBNT 24V POE adapter?
Yes it can
http://routerboard.com/RB951G-2HnD
The power between 8-30V all can it is?
What it means is that RB951 can support power from 8 to 30V, and if I am correct all routerboards do with the exception of RB800. So you are safe to plug a 24V power supply.
Yes, and check this out hire come the best SXT HG can work on 6V-30
It can work starting from 6V, but it doesn’t mean it will work with a 6V PoE injector, because the voltage drops with longer cables.