Definitely. Worrying if your stack of choice scales is premature optimisation. Get up and running asap, fix scaling issues if you ever have the luxury of running into them.
You bring up a valid point, there are always exceptions to hard fast generalizations. For example a web company is putting itself at a disadvantage by adoption C++, even if their developer is most proficient in it. The stack has to align to the domain or else your developer has a lot more work to do to get to the finish line. Certain stacks have so much inertia in particular spaces that the existing libraries and API's more than make up for the reduction is efficiency of a developer that is more comfortable in other cloths. C++ for web being one or VB for embedded being another. Choosing the right tool for the right job, should always be taken into consideration.
True fact. palish is wrong.
I dislike C++ but develop WebApps in C, so what?
My stack supports it and I'm good at C and still can use all other language libraries installed in my system. Clap Clap.
If your co-founder is "lightning fast" at it, that is exactly what you should use.
Down the road, if the .NET stack is not suitable, you can always migrate. But why worry about it now?