1) It depends on how you want to set it up. If I'm NATing my guest traffic to the AP, I set up the rules on the APs. If I'm dropping off to VLAN I setup the rules in a Group Policy on my MX associated to that VLAN.
2) Wireless would be processed first then MX rules if you had both.
3) As wireless is processed first that would block traffic to any RFC1918 subnets.
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