I wasn't really sure where to post this.

I have some legacy Mac Servers running Sierra Server (10.12.6) and FileMaker Server 13.

I run Sierra on a MacBook to manage them.

I'd really like to upgrade the MacBook to Catalina.

My issue is, I know FileMaker Pro 13 won't run on Catalina and I know FileMaker Pro 18 won't access databases hosted on FileMaker Server 13.

What I don't know is if there is an interim version for FileMaker Pro that will run on Catalina and open FileMaker 13 format dbs.

Anyone know?

Only FM18 is compatible with Catalina (https://support.claris.com/s/article/FileMaker-and-macOS-Catalina-Compatibility?language=en_US), so: no.

You could virtualize an older macOS or install Windows 8.1 or 10 in VMware Fusion or Parallels to run the older FM.

That is one solution but I fear it may be swallowing the spider to catch the fly.

What can I say... you do what you have to do in order to support your customers.  Your conundrum is not unique.  It's why most of us developers that use macOS as their primary platform have to be careful updating their own OS.   I use VMware Fusion extensively, I even have my own VMware ESXi on an old Mac Pro to run older OSes plus I use AWS Workspace as a Windows machine so that I can keep supporting whatever customers bring to the table.

It's the cost of doing business.

Ehhh, yes, and my current solution is to maintain a Sierra machine.

The question is about compatibility ranges and if they can be leveraged to allow compatibility between FMP 13 and Mac OSX 10.15. For example, can FMP 14 run on Catalina and open FMP13? 

If you don’t have any input on this then I guess we’re just pissing in the wind.

See the first sentence of my first reply.  There is no compatibility range if you upgrade that machine to Catalina.