Me no expert, but..
The "guest" network should be outward bound only. No access to the staff/servers internally. So it will be on it's own subnet, supporting up to about 250 guests at a time using common router firmware.
The "staff" network probably needs to be bigger than 8-bits. So if you make it a /16 subnet, it can then handle up to 65000+ simultaneous IP addresses. The only question is, "can the existing DHCP server do that?" Common wifi router firmwares are hardcoded to /8, so they probably cannot do it, or not with their default firmware.
So find out the limitations of the gear first.
Cheers