Guest network on RT-AC68U (and a question on AP isolation)

Hi!
This is my first post here, sorry if this is the wrong forum.

I have a few questions regarding the guest network feature:

1a. Are devices on the guest Wifi network protected by the same security features as the main (wifi) network? Is it as secure? If not, what differs?

1b. Is for instance the firewall (towards the Internet) enabled for the users on guest network aswell?

2. How many guest ssid can be run at the same time without sacrificing stability or performance?

3. I am...

Guest network on RT-AC68U (and a question on AP isolation)