I started reading about co-channel interference the other day and noticed that the Meraki APs have auto channel selection capabilities. I'm wondering now that if there are existing Meraki APs already installed in the floor/building will they affect the newly installed Meraki APs giving off completely different networks, SSIDs, etc. ?
All other 2.4GHz and 5GHz traffic seen by a Meraki AP influences it's decision on channel choice and radio power. You can fix both if you are having real issues, though this can cause other problems. I find that using a DFS channel on 5GHz is often the best way to get some 'clear air'.
Yes, all other Wi-Fi access points and 2.4/5 GHz devices will be considered by the Meraki access points when choosing a channel. Typically leaving the Meraki on automatic (the default setting) will be the ideal setting. Nearby access points on the same channel will typically "share" the air time, nearby access points on overlapping channels will typically create noise and "interfere" with each other, and nearby access points on different channels will not impact each other. If you'd like to avoid interference, you can manually assign channels per AP, or you can configure the Meraki RF Profiles to avoid specific channels.
For example, if you have an access point from another network (Meraki or not Meraki) using channel 36 and very close to your access points, I would recommend using RF Profiles to avoid channel 36.