That's up to you. It's generally acceptable to use Pokemon without the typing you're using as long as you plan to evolve it into a Pokemon with the typing you're using. In this case, as Slugma evolves into Magcargo, it's within the rules. However, it's generally not allowed to have Pokemon with the type evolve and lose the type, like Onix to Steelix.
There's no definite rules, though. This is your playthrough, and you can choose what you want to do to make it more enjoyable. Using pre-evolutions without the typing could be considered "cheating", but it also gives your team more variety.
Hope I helped!