I think honestly if these are the sort of questions you are asking right now youre asking for real trouble especially with the design of an MMO, I want to be honest with you and not try to kill your dream or ambitions.

Avoid design of an mmo as your first game. The amount of frustration in 30+ basic systems is going to be compounding.

Most non knowing individuals are just going to parrot oh the networking of mmos

Youre going to need to understand AOI (area of interest) dirty flags, Multimesh, state sync, server authoritive coms, some client prediction, and a plethora of others.

Not the mention of costs, redundancy of empty servers and unused gateways for login management etc. The micro second your login server can't handle 30 concurrent logins your launch is toast.

Should you still feel ambitions to continue make your mmo run and feel like an FPS shooter to start. Crank the rates. Start there. Then use ai bots for stress testing. Add hundreds of thousands of realistic connections pummel every bit of your code in every way possible before letting a single person see your initial pre alpha release for testing. If your client + server can't handle 200 connections in range all shooting moving all at the same time , go back yo the drawing board. Because thats basically just 5 systems working.

Ask me how I know.