For a general TOS character use in Disco(inferno). I for one have no problem with it as long as its not a gratuatis "oh hey" type of thing, thats fine. If Pike, or someone wants to make a quip about how an ensign, Lt. or cadet Kirk done this or that as an easter egg, thats fine.
If they want to have McCoy come on board as a low level doctor, not CMO, they can. Have them mention Scotty in trying to solve some technoboble. thats okay, there both in Starfleet and doing there job. Now, Checkov is like 15, so nope. Uhura is maybe a cadet, same with Sulu, maybe Tilly mention them or something, but thats about it.
Now, Have Captain Deker, or a Captain Nogura, or some other admiral or captain from Tos pop up, thats okay, there out there right now, doing there thing, if they get mentioned, seen on screen, even pop up on the ship, I have no problem with it.