In my experience (over 25 years of computer maintenance, repair, and building), ATI's (now AMD's) Crossfire platform is designed so that any Crossfire capable card can be linked to any other CF capable card. HOWEVER, I would recommend that the cards be of a similar generation - R series paired with R series, HD series paired with HD series. I personally have paired an HD4350 with an HD5750 and had no problems. I won't say pairing an HD with an R7 or R9 WON'T work, but you should have no problems since you are pairing an R7 with an R9.
Anyway, it's a pointless endeavor if you aren't putting them into a system with an AMD APU (Accelerated Processing Unit) in a motherboard with an FM2+ socket because they WILL NOT get the full benefit of their PCI-e Gen. 3 capabilities. If the MB is CF capable, they shouldn't have any problems, though.