Verified answer

the partner you choose has nothing to do with the evolution your eevee will take. eevee is a special pokemon in that it mutates easily. the word eevee is actually based on the word evolution. in diamond and pearl, there are two new evolutions, a grass type, leafia, and an ice type, glacia. both are likely going to be renamed leafeon and glaceon.

in mystery dungeon, your partner has no bearing on their evolution at all. besides, it just wouldn't be smart to have two fire types, or two electric types - because what one is weak against, the other is as well. it's always good to pair things up that can fight each other's weaknesses

you can't evolve until after you "beat the game" anyway - at which point, you don't need to have your main character or their partner actually go on any missions anymore. my team was an eevee and a bulbasaur at first, but since i've beaten it, my team leader is a crobat, and it's partners are a pichu and a girafarig

in order to evolve eevee into espeon, you need a sun ribbon, found on floor 20 of wyvern hill, in a locked room

in order to evolve eevee into an umbreon, you need a lunar ribbon, found on northwind field, on floor 20

in order to evolve into vaporeon, you need a water stone, found on floor 29 of northwind field

in order to evolve into flareon, you need a fire stone, found on floor 29 of fiery field

in order to evolve into jolteon, you need a thunder stone, found on floor 29 of lightning field

Charzard