Can women and men be counted together for a minyan for megillah reading for the purposes of saying הרב את ריבנו?
You mean would it be different than Rema (Shulchan Arukh OC 690:18) regarding the preferences for 10 where he writes – הגה: ויש להסתפק אם נשים מצטרפות לעשרה? Or are you asking whether we rule that way? I would say that the halakhot of berakhot and preference for 10 in general should definitely be the same. As to do we rule that way – one could argue ספק ברכות להקל, but also see Biur Halakha 692 ד״ה אלא בציבור who quotes shitot that you can make the berakhah even ביחיד. Given the larger issues here in general – an opportunity to count women as part of the ten – I would definitely say we can and should.