I am a little confused about fire alarm monitoring services.
FCI fire alarm panels list what receivers they are compatable with.
What companies have limitations on who can monitor their systems (if any)?
Specifically can Simplex be monitored by any service?

There can't really be a limitation. The monitoring service is often specified by the city. I am on an E-911 board and we have just started to switch over to a radio monitioring service. We have a reverse polarity city tie system now. The building owners have to contract with our specified service to buy or lease the transmitter that will be connected to their fire alarm system. Our radio system vendor is also a licensed fire alram contractor and has assured us that they can supply a radio transmitter for any fire alarm system. The owners have about a year to make the switch. After that time, the reverse polarity alarm equipment will be removed from out dispatch center. A number of building have already switched to the radio system and some of them are Simplex. The vendor had no issues making them work. We are using AES-IntelliNet equipment to connect the buildings to our central dispatch center.
Don