Zkweelur is talking about a very old pack, Pokémon Essentials Online, that have the start party of mew,celebi,jirachi,manaphy,arceus,giratina. I know almost nothing about this pack, but in default Essentials use pbAddPokemon to add a pokémon instead of this sample party generator. pbAddPokemon(PBSpecies::TREECKO,5) add a level 5 Treecko. With basic RPG Maker XP skills you can create a starter system with this.
It's not usable in the current essentials.
Also, Pokemon Essentials Online is unstable, old and I recommend you don't even bother with it.
I might release an online starter kit myself, if I feel like it.