Internet access for for Guests network can simply be throttled so main network will not be impacted by heavy users visiting your house.
First we create a limiters for "In" and "Out" traffic using a Traffic Shapper:
Firewall -> Traffic Shaper -> Limiter



Click on "In" and click on Enable, give it a name and choose the bandwidth that will fit your needs:



repeat the same for "Out"



Then go to Rules:
Firewall -> Rules and choose "Floating" rules:



Create rules for "In" traffic:



Select proper Interface, direction, Internet protocol. It is best to use Alias for Source to properly target IP's or network.
In our case we have selected every IP connected to the Guests interface.



in advanced features select proper queue order. Keep in-mind In/Out reversed there.
Similarly create "Out" rules:



Do not forget to apply changes for rules to take effect.There's no place like ~

Note on Interfaces:
LAN - Home Office network has access to IOTS interface and has Internet access.
IOTS - has access to only one server on LAN. No Internet access
GUESTS - Has no access to IOTS or LAN networks, only throttled Internet access.

Here is LAN Speed Test:


and this is GUESTS throttled Speed Test:



It is working!
Tests performed on the same computer VIA WiFi.There's no place like ~