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

[mattLAN90]

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 thinking of using a guest SSID myself to isolate my devices from devices connected to the main wifi and through LAN cables. Will this work?

Abit Off topic, I have a few questions on AP isolation:
Let's say AP isolation is enabled on 2,4ghz and 5ghz.
Connected to 2,4ghz main wifi (not guest) is a Windows Pc and a smartphone. Is it correct that these devices won't be able to speak to each other?

Let's say that a MacBook is connected to the main 5ghz wifi. Will the Pc and the smartphone on the 2,4ghz be able to speak to the MacBook, despite AP isolation on both the 2,4ghz and the 5ghz

I am running the latest stock firmware.
Thanks in advance.