1) Are you using Antminers or similar network based ASIC devices, or is it GPU mining? The Remote Agent is only intended for GPU mining.
In any scenario, you either need to setup a VPN solution (Hamachi or similar), or configure port forwarding. You need a public and static IP in the port forwarding scenario at least. With VPN solutions like Hamachi, it might work without.
This is a guide on the web site on this topic where you will find more information:
http://www.awesomeminer.com/help/remotenetworks.aspx
2) Yes. You can select a single miner, multiple miners or all miners and perform the Switch Pool operation. This will let you change pool for all your miners at once.
It's GPU mining, a custom 6 GPU for non bitcoin mining. I thought the agent already automatically port forward and listen to the socket connection. That's mean I can use no-ip or dydns as the external IP for my dynamic IP problem, correct?
Yes, you can use any of those DNS services for solving the dynamic IP issue.