Hi John,
I would say 8 inch would be enough to break into this game. Claudio Balcon who is now the leading amateur in this area started successfully using a 0.2m aperture with his similar home built 200l/mm grism slit spectrograph, though he uses a larger 0.4m aperture now.
https://www.wis-tns.org/search?&page=1&classifier=Claudio%20Balcon
(The ones before August 2023 are all with the 8 inch)
I work down to around mag 17.5 normally but there are quite a few candidates that get to around say mag 15.5-16.5 before being classified which should be within reach provided your skies are not too badly light polluted. An 8 inch at f7 is a similar focal length as my C11 running with as standard 0.63 reducer at f5.5 so if your seeing is similar it should match the same 23um slit I am using.
These are my classifications, most with the ALPY200. I tend to do more follow ups than initial classifications these days but the most recent 2 are official classifications, one at ~mag 17 and another ~mag 15.5 with strong contamination from the galaxy
https://www.wis-tns.org/search?&classifier=leadbeater
You’ve probably already seen it but I did a Youtube presentation on it for Shelyak when they brought out the commercial version
https://www.youtube.com/watch?v=0L6LLn9HjUY&ab_channel=ShelyakInstrumentsTV
Cheers
Robin