A pair of 5510s will be able to creae a failover pair as long as you have the same installed memory and software revision and hook up the interfaces per the design guide.
I'm not sure why one would report 1599 and the other 1600 Mhz. I suspect a minor hardware (or possibly firmware) revision that causes this primarily cosmetic difference. I would be confident in setting them up as a failover pair.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...