This is why I don't think it would be the best idea to make an Eeveelution for each type. A "Toxeon" would seem redundant given Umbreon's poison references.
Instead, I think it may be better to create new evolutions (or temporary forms) for the Eeveelutions, adding to them the unused types. Umbreon's evolution/form could be Dark/Poison, Vaporeon's Water/Dragon, Leafeon's Grass/Ground, etc.