The OMS setup follows the studio - NOT the app (that's why it's called "Studio Setup" - duh)
It describes ALL of your MIDI devices, interfaces and interconnections.
You're supposed to be able to make ONE setup that works with everything - regardless of which app, interface or modules are actually in use.
OMS's job is mainly to sort out and remember MIDI details like channel assignments, routing between MIDI interface and however many keyboards / modules etc.
The main reason for having the ability to select from multiple setups is portability. If you say, have a multitracked sequence recorded and you move the computer to another location/studio, OMS will recognize that stuff doesn't match what was used to record and play that sequence so it will list all required channels and modules and offer to help you remap the tracks to substitute destinations in the "foreign" studio.
It also comes into play if you say, update some equipment and/or change channel assigns on the new stuff. When you launch a previous sequence, OMS setup will pop up so you can reroute/reassign the tracks to the new stuff.
* Point is, you don't need more than one setup at a time in one location unless you say, have too much money, you own a Peter Gabriel - sized pile of MIDI synths and samplers and you use multiple setups to address different roomfuls of stuff.
* Personally, in the century or two I've been using MIDI, I've never needed more than one setup at a time.
* All that said, adding Recycle and Alchemy (!) combined with mule-headed old Akai and Roland samplers MAY require you to get creative in unpredictable ways to keep everything orderly. You're not contemplating an everyday configuration but a rather unusual one (with ancient Powerbooks, no less). Sending MIDI data to play samples is one thing, getting and sending sample dumps over MIDI is another. Doing that over all of that early-MIDI hardware and software AND synchronizing with audio (on a Wallstreet?) will surely require time, experimentation, hair-pulling and……? The only way you could make this more complicated would be to try to also hang a SCSI sampler on the weird Mac HDI-30 port.