Grass and Steel resists Water and Electric without being weak to Ice or Grass. I think it's the only type combination to do so. It also resists the Dragon+Electric and Dragon+Water type combinations. The Fighting weakness doesn't seem that bad to me since Machamp will confuse you if you're not a Ghost-type and a +2 Close Combat from Lucario will OHKO almost all Pokemon that don't resist it. Gengar's Focus Blast would mean that it can't counter it but since many Gengar run HP Fire anyway, you shouldn't be switching in on one unless it's locked into a resisted move. The 4x Fire weakness isn't great but you can usually predict Fire attacks, anyway.
That said, I'd much rather see Tsutarja become Grass/Electric. We know Tsutarja has the lowest Base HP out of any of the starters so it's going to be pretty frail and wouldn't be able to benefit from the defensive nature of Grass/Steel. Grass/Electric is a much better offensive type since it's only resisted by Grass-types, Dragon-types, and a few other type combinations like Electric/Steel, while Steel offers very little in combination with Grass.