Usually talking about a mixed cluster means that you run ESXi hosts with different versions (e.g. during an upgrade).This will usually work since the vCenter Server pushes the new FDM to all hosts. (see http://frankdenneman.nl/2011/10/fdm-in-mixed-esx-and-vsphere-clusters/)
vMotion/DRS (live migration) is not possible between Intel and AMD CPUs (or any CPUs which have no common EVC baseline). HA may still work, because this will not do a live migration but restart the guest on another host. However, you may experience some issues in such a case in the guest OS, which also needs to recognize the other CPU.
André