2 2nd gen ipad, 1 ipad air 2, iphone6, and samsung s5 the air/iphone6/s5 should all support ac no?
Yes they should. If you haven't already done this (as a test) put your 5 ghz at 80 mhz (not 20/40/80). If you have it set on "auto" switch it to N/AC mixed (or vice versa) and test those 3 clients again.