Arc SDE 32 and FME server 64 bit with 32 and 64 bit engines

Basically you need to assure that the "bit-ness" of all the components running on the same computer as FME are the same. This means that if you're using FME 64-bit, then all the associated components, such as ArcGIS, Oracle client, external Python interpreters, etc, also need to be 64-bit. (This is not a limitation with FME, but rather of the Windows OS architecture).

Software that is running on a different machine (e.g. Oracle server etc) is not affected by this.

The authoritative guide to FME and ArcGIS compatibility is here:

https://knowledge.safe.com/articles/1517/notes-on-fme-and-esri-versions-and-compatibility.html

It may not be the most obvious article, but it's really important for understanding what works together.