I don't know of any server that allows you to supply a target list that large for a single message.
As for the 'multi users private chats', as I said before they already exist in IRC. They're called channels. There's absolutely no difference except channels can be more customisable if you choose.
The WATCH system is a more advanced version of notify to replace the standard ISON method, it removes the need for clients to continually query the server for the status of the list. You should try some different networks and see. Servers which have WATCH support will usually have a WATCH=N token in raw 005 (where N indicates the limit of many nicknames can be watched at once).
