"IP Group (By Users): Select this option to group traffic on each WAN interface by priority levels or classes of service (CoS). With this feature, you can ensure bandwidth and higher priority for the specified services and users. All traffic that is not added to the IP Group uses Intelligent Balancer mode. To specify the services and users, click the Edit icon for the WAN interface and then add protocol binding entries for each service, IP address, or range of IP addresses."
This would lead me to believe that you can accomplish what you are wanting to by using CoS and IP Group.
As for the ISA question. I'm a long time user of the ASA, and the PIX before it. The ISA is not a replacement to the ASA in any fashion. It's directed at a different target audience. That said, I've been very impressed with how functional it is. There are some things it can't do (like NONAT). However for the most part it's very similar to the ASA in how it uses Address/Service Objects/Groups, NAT/PAT, Static NAT, Access Rules, etc. It's doesn't have a CLI. It doesn't have an Active/Standby option however it does support VRRP. To answer your last question, it doesn't have a modular capability to support expantion. As I said at the beginning, I've very impressed with it though. You can take a look at the emulator if you want. It's located here:
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