Yes, they'd be too short.
I'm about the same weight as you. I have one pair of skis that are similar to the im77s -- 176 Fischer AMC79s (79mm waist). My other skis in regular use are 186 Fischer Big Stix 84s (84mm waist), 188 PM Gear Bro Models (99mm waist), and 195 Praxis Powders (136mm waist, reverse camber and sidecut). This year, I also skied 191 Scott P4s (108mm waist) as a conventionally-shaped powder and soft day ski, but I'm replacing those with 190 Moment Rubies (112mm waist) for next season. Note that all of those other skis are significantly longer than my AMC79s. That's not an accident.
There are a lot of reasons why you'd want to ski Mojo 90s, in particular, longer. First, they're twin tips. That means you'll have a shorter effective edge than a similar ski with a flat tail. Obviously, the amount of kick in the tail varies, but figure you should add 5cm or so. That puts you at least into the low to mid 180s.
But since you're looking for a powder/off piste ski that can also handle groomers, the good news is that there are more options today than there have ever been. The question is what you want as a two-ski quiver.
I'm going to suggest that what you don't want to complement the im77s is another pair of skis that are conventionally shaped and only 13mm wider. Those would be replacements for the im77s, not complements.
There are plenty of great, wider skis that do great on groomers. The Moment Rubies are a great example -- I fell in love with them on groomers that were softening in the spring sun, where they were like a freight train.
I'd encourage you to try demoing some skis. Try a pair in the 90mm width range as a baseline. Try something 100 or 110mm wide to satisfy yourself that you won't sacrifice much in groomer performance by going wider. Try a few different constructions in the larger skis -- more vs. less sidecut, more vs. less camber, stiffer vs. softer -- and get a sense of what you like better on the groomed and the soft.
Then come back and we'll find you the perfect ski. ;-)
Alternatively, if you find a smoking deal on something 100-120mm wide, probably with a radius in the 25+ meter range (because sidecut matters with soft snow skis), buy it and try it, and if you don't like it, sell it at a slight loss to someone who will like it.