Yes but "Percentage Based" and "Failover host" will ensure your VMs can fail-over at any time by setting aside the required memory overhead reservation. Now, admission control is is no resource management. So if that is what you are looking for, the fact that VMs get the same resources after a failover as before, then you will need to use VC Ops and monitor the environment closely.
cragdoo wrote:
Therefore HA will utilise the default slot size, which will be a 'false' value, as the number of VMs I can power on will be significantly lower than the number of available slots.
Actually the number is exactly the same from an HA perspective. HOWEVER, you are talking about a "usable" situation and resource management rather than failover guarantee. Some customers manually set the slot-size to a give number, for instance they always want their VMs to have a bare minimum of 512MB available, so that is what they set the memory slotsize to.