Boosting poison and steel to have another thing they're good at hitting? Yep, that was on the checklist. Building in a counter to dragon which it really didn't have? Check. Yup, there's some needed balancing in there. (Grass is still screwed though.) Fire gets to be a little better than it was, steel gets something it can actually hurt, but fight, poison, psychic, dragon get nearly balanced, and dark gets weakened a smidge.
Sounds like a pretty good balance from one type to me. Normal, grass bug and ice are still screwed, and steel still has a mega surplus of stats due to how many things its strong against, but… its better balanced.
Normal -4
Fire +1+1=2
Water +2
Elec +1
Grass -5
Ice -3
Fight +1-1=0
Poison-2+1=-1
Ground-+2
Fly+1
Psychic-2+1=-1
Bug-3
Rock 0
Ghost +1
Dragon+2-2=0
Dark-1-1=-2
Steel+7+1=8 (Though it wasn't very offensive before)
Also, dual type attacks sound fine. it'll probably only apply to a very small handful of actual moves, similar to special moves that hit the physical defense stat, or the teamwork moves. Also means it'll give moves more coverage and critters that get stab with them... so any move that is dual types probbaly won't have very good stats and that's the tradeoff.
Also, Mawhile getting a fairy type is interesting especially since it's already steel type, so that'll counter some of the weaknesses, with the immunity to poison.