SPECIFIC ANSWER for your question:
Q: Does that mean I cannot use some pokemons as a follower in POKEMMO?
A: No you can, there is a popular mod called Sparkling Shiny Follower Sprites that is made by Billla and Mortedesu from the POKEMMO forums.
Sparkling Shiny Follower Sprites is a much better/updated version than Billla's older mod, which is the jump version. Mod link:
Q: How to install the mod?
Q: Is Everyone else not fine with it?
A: With what PokeMMO can do, and knowing pokemon fans, they wouldn't like it very much. If only Pokemons from Unova region cannot follow their character, the game would feel incomplete. Even with the new Pokemon version, Pokemon Sword and Shield, there are those people who still wonder why Gamefreak developers did not add the lovely Pokemon follower feature.
_ _ _ Slightly Longer Q&A: https://www.reddit.com/r/pokemmo/comments/d0p02y/how_to_make_pokemon_follow_you/fi6ohkb?utm_source=share&utm_medium=web2x