Hi
There's a long answer to this question that is best discussed with your local CP office Maestro expert, but the short version is:
Dual site security groups should mirror each other in terms of hardware, as this ensures proper HA as both sites have the same capability. If you do want to mix different hardware in a security group we can do that, but there are limits as to what's supported. Details are in this SK. Even if you mix models, the two sites should mirror each other in terms of SGM makeup. Details on how mix and match work are in the admin guides.
In short, it's not supported to do a dual-site security group with 28600HSs on one side and 16600HSs on the other side, even if you add more smaller appliances. More small appliances do not scale the same as fewer large appliances, especially when it comes to VSX due to the nature of the scaling - all VSs are active across all SGMs, and will share the same CoreXL per VS config. Hence, if you have a VS with 4CXLs configured, they will have 4CXLs per SGM - so if one site has 2 SGMs and the other 3, the VS will have more instances running on the site with more (smaller in this example) appliances.