Of course being able to win with size, strength and wearing teams down is ideal. I don't think having Rossi on your team means you'd have to play a completely different style. Boston's a good example of a team with a mix of smaller and bigger guys. They can beat you with skill but also aren't easy to play against. Those smaller guys need to have that high level of compete and a willingness to go to those tougher areas which I think Rossi does.
In this hypothetical, did Ottawa pass on Stutzle/Byfield, or move Stutzle to the wing? Otherwise, if you go with that #3 pick as your top centre, Rossi could be your #2. Your first post made me think you like him offensively but it's his defensive game and playing against other top centres that scares you off. Would you be more okay with him in that #2 role? Especially with all the big players Ottawa has in their system already (Tkachuk, Pinto, Formenton, Batherson, Norris).
What I think will allow Rossi to succeed, despite being 5'9", is his strength, intelligence, mindset, and competitiveness. His skating improved a fair bit from year 1 to 2 in the OHL, so that's something I hope he can take even one step further, making him an even tougher player to play against.