Why not just tone down the difficulty of combat encounters so the extra GMPCs aren't needed? I mean, as GM, everything is up to you so toning down the difficulty of combat is a simple choice you can just make. I once ran a decent length campaign for a single player playing a single PC, and not once did I need to have a GMPC in order to make combat survivable. As GM you control how the imaginary world works, you can easily make combat survivable for a party without needing extra GMPCs, it's a trivial thing to do!
If the GM is concentrating on making combat survivable, what is the point? The GM is just handing out participation trophies.