Definitely, without doubt, put the stereo cartridge on the long arm. Pick an arm with removable headshell if possible, gives you flexibility now and in the future.
Does your 9" have a removable headshell?
My 12.5" long arm is fixed cartridge, used for my MC cartridge. Only change it when worn.
My 9" has both removable headshell and extremely easy arm height adjustment, so it is most flexible. I have several cartridges I alternate, MC, MM, and now an alternate Mono. And we play my friend's cartridges on it. SUT handles various MC impedances, and has Pass for MM.
I added a 3rd arm for mono, fixed. Limiting, but my 9" arm saves the day regarding flexibility.
My Grado Mono is elliptical, on the 3rd arm. I'm very happy with it. I'm not looking for perfection, or even 'best', just gaining the advantages of playing mono lps with a true mono cartridge.
I just had VAS put a new boron/shibata on an AT33PTG Mono Body. For modern Mono, Haven't used it yet. He played it at his shop, we all were impressed, no comparison here yet.
3 arms: MC, MM, Mono all ready to go in seconds in any listening session.
The compliance of the cartridge and the mass of the tonearm must ’avoid a problem’. https://www.ortofon.com/support/support-hifi/resonance-frequency/
Today’s equipment, I would simply put my existing stereo cartridge (likely high compliance) on my new long arm (likely not too heavy) and expect no problem.
Perhaps, before you buy it, pick an arm you want, tell the members here about it and your cartridge, get their help.