Is it possible to set ether5 to Master and the rest to slave of ether5?yes, all 5 ports are equally configurable on a RB750.
the ether1 port can't be switch master, that's the only limitation. you can put any config on the ether5 port.
I looked up "rb750.pdf" on routerboard.com and the line below is written in it.Only ether2-ether5 can be switched on RB750.