You probably spoke to someone who didn't know what Sidewinder was.
Sidewinder became McAfee Firewall Enterprise after v6 (Sidewinder G2), but those of us who have worked with the product for that long still know it as "Sidewinder".
Officially McAfee always encouraged HA clusters to be made up of the same hardware appliances. But, fundamentally, as long as each has the same number of active interfaces and the zones/burbs are created in the same order on each appliance then you should be able to create an HA cluster.
Mixing appliances could be tricky if they are of different performance specifications (e.g. using an S3008 as the main appliance and teaming it up with an S1104). When a failover event occurs is it likely the users will notice a drop in performance. Also, with the S1004 only having 4 interfaces you would only be able to use 4 of the 8 interfaces available on the larger appliance.
Ultimately, I have always recommeded to customers that they use the same appliance (or direct equivalent) when setting up HA clusters.
As far as I know regarding the Stonesoft NGFW product, this will be able to run on the S-model appliances, but not the older ones.
Hope that helps.