K39543431: Configuring an alternate disaster recovery device in conjunction with an HA configuration

Danger will robinson -danger.... Its probably less about can you do it - and more about should you do it.
Each version train of OS has its own needs for minimum specs, and you typically need to add a little more from the base if you are using more than 1 modude (ltm or gtm -   vs ... ltm/awaf /firewall )

The HA / cluster components are tied to version train, in order to maintain sync- and failover the VERSION needs to be kept the same  .... 14.1.x.y  and 14.1.x.y, upgrades to 15.1.a.b and until  the 2nd unit gets upraded sync can not happen.

from the hardware prospective, you would want simiar size equipment -but the exact model should not be an issue so long as it have the right capacity and config.