If SG is in L3 mode you just creat a new VLAN cameras with any IP you like. Tell camera vendor to use that IP as default gateway. Assign a single access port in that new VLAN and plug camera switchinto that port.
I am not clear about your DHCP issue though. My SG is in L2 mode so I don't see the same options. I would think you could have a dhcp pool per VLAN.
Also, why let camera vendor choose IP range? You should tell them what they will use..
Good luck.
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