Two Operating Systems on One Mac?

Well, no external will be as fast as an internal, so yes, it will be a bit slower, but considering the fact that a fusion drive only allows 2 partitions and Catalina now creates 2 separate volumes (whether you like it or not), I would not feel comfortable enough to try partitioning a Catalina fusion drive. With Catalina, you now have 2 volumes: Mac HD (read only) which contains your OS/system and Mac HD - Data which is read/write and contains everything else. My experience with the fusion drive was not the most positive since it took me 2 days to get it back to factory fresh condition (I was returning it to Apple because I didn't care for a couple of other things as well as the fusion drive). The only thing that finally worked was to destroy (= erase) the thing booted into recovery and then recreating it. And Catalina makes it a bit more complicated with its two volumes. Apple is trying to cut anyone off from accessing system files, a new security feature. The side effect is that you can't do anything either.....

In the US, OWC is the most recommended for RAM, hard drives, SSDs, etc - they a Mac centric and they do ship to other countries (this is: macsales.com).

Look at Partition a physical disk in Disk Utility on Mac - Apple Support

It seems recent macOS is more tolerant about on-the-fly resizing of volumes, etc., though you should still back up in case something unexpected happens.

I have not memorized all the details of all Mac computers. If yours came with Catalina it cannot run anything older. If it came with an older OS then it can boot to an older OS but it will have to be on a separate drive volume. You can set this up in Disk Utility. The only thing is (and I can't help you here), Catalina actually has two volumes and I am not sure if doing this on the same drive will start making complications. Somebody else can advise you better but you may wish to consider booting off an external drive.

I have an older MBP (purposely) which can boot into its original OS (10.9) which is the last OS version where my movie apps will work flawlessly. However, the steps I took were: 10.9 was on the MBP when I got it; I cloned that to an external drive with CarbonCopyCloner and will boot into it whenever I want to work on a movie. My internal drive has been updated.

So, you can put two on the internal, but that would mean that you may have to erase your drive completely. That means you'd need to make a (preferably bootable) clone on an external; boot into that, wipe your drive, re-partition your drive to two partitions, at least one being Mac OS Extended (Journaled) and GUID partition scheme. Then you can clone your current OS back onto one partition and, using recovery (or a clone) put the older OS on the Mac OS Extended partition.

Having a fusion drive makes such things much more difficult - it does not like being partitioned although it will allow a total of 2. I did that once and it took a couple of days to undo it - not worth the effort. So, my takeaway is: do not mess with fusion drives.

A simpler method would be to clone your older system to an external (formatted correctly) and then plug that in and boot from it when you want to work with Filemaker.

All of this assumes that your Mac can boot from the older OS (meaning it used to come with it or was upgraded to it).

Use Parallels or VMware to create a Guest macOS operating system on an external SSD in a USB3.1 Gen 2 enclosure using a USB-C connector. Give the guest 8GB RAM, and assign at least two cores, but not more than 4 cores to the guest. All Parallel's needs is to point it at the macOS full installer, and install the Parallel's Tools after installation is complete.

I am using an external drive as described above (500 GB Crucial MX500) as the repository for the Paralle's El Capitan guest, and it works fine on a Late-2013 iMac with 16 GB RAM and an internal SSD. Should fly on your Mac, and avoid any partitioning concerns altogether.

The trouble with booting from an external drive is that it's much slower.

That was true of the old USB and Firewire; the newer ones are much faster.