All competitive grade mono teams have done these 4 things:
- Get dual typing/ability/move that covers weakness(es).
- Variety! Don't take 1 to an extreme... that would be stupid.
- Look up the worst Pokemon for your team to go up against. Expect to see them, and know how to counter them. Sometimes preparing for a few specific enemies (like in a tier and/or game version) is wiser than preparing for an entire type.
- Take advantage of your mono type's benefits. E.g. If you are making a mono fire team, then having drought/sunny day is necessary.
You are using this ingame which means the 4 rules above are helpful but not necessary. If you challenge the champion of whatever version you're playing with 6 non-competitive grade level 100 fire type Pokemon, I promise you will win on your first try. In-game mono teams are for fun, not power, so choose whatever you think looks best. Personally, if I were to choose the best mono team ingame it would be steel because it has many resistances that dominate early game, they generally evolve at decent levels (unlike almighty dragons), and steel types tend to have good attack stats by end game too. Again, without knowing which game you're playing this can change since steel wasn't introduced until Gen 2.
Here is a link to some competitive mono fire teams in case you're looking for some inspiration: http://pokemondb.net/pokebase/rmt/search?q=Mono Fire Team