Cisco Clean Air is RF interference detection and mitigation solution, it can provide real time visibility in to RF spectrum in your premises so you can easily identify any RF interferers in WiFi spectrum, for example lets say you have some interference in 5GHz channel 36, depending on the configuration WLC will automatically change the channels on the neighboring AP's, so your wireless infra is working on clean channels. Or due to the visibility it provides you can take action manually as well.
Its not compulsory to have dedicated Clean Air Sensors (Monitor mode AP's) to get the clean air full feature set working, but you need to have clean air capable AP's, you can have the client serving AP's for example new Cat9120, 9130 with RF asic can scan the spectrum while serving the clients. while the older generation of clean air capable of AP's are more than capable of detecting and mitigating interferers, Cisco recommends deploying dedicated monitor mode AP's as this can provide spectrum intelligence with off channel scanning and due to dedicated radio used for scanning more visibility. Information collected by Clean Air capable AP's are vital part of the RRM, based on the informaiton made avaialbe to RRM by Clean Air it chooses best channels where the AP's can operate on at a given point. Clean Air is a big topic which is very difficult to summarize. You may refer the below for more information.
https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/config-guide/b_wl_16_10_cg/cisco-cleanair.html
Currently we have one AP per location and each AP is in a separate WLAN with its own VLAN. Given that these locations are pretty far apart would it be feasable and/or a good idea to merge these all into a single network with a single SSID so if someone is traveling in the company their devices will link up at all locations with no additional config.
This completely depends on your organization policies, My preference would be to maintain a Single SSID organization wide, so it will ease the management burden. Again based on your organization policies and security requirements it is up to u to decide. Design phase is the key here.
And finally can someone link a guide or reference to setting a guest network that is isolated from the internal network? I will be honest that I have not even done a cursory google search on this issue yet as it is a back burner issue for me at this time.
Guest deployment depends on the hardware you have, since you have multiple sites my recommendation would be to consider a Anchor and Foreign controller design. This way you can directly terminate your Guest traffic in to a DMZ. If you are using new 9800 controllers anchor controller can be optional as you can utilize a trunk interface directly connected to a DMZ segment. If you have Flex connect AP's you can have your Guest SVI directly on a Firewall or any upstream L3 device which doesnt allow traffic towards your LAN.
