I was told by someone (non chef) that if I am out of ie. cake flour and in a bind, to take all purpose - per 1 cup, take out 2 tbsp and replace with cornstarch. Same "rule" seems to also apply for pastry flour, bread flour. I can't see how this substitution would work across the board. Thoughts?
It doesn't work across the board. Rules of thumb like that lead to approximation and acceptable substitutes, but not necessarily the exact formulation of the real item. That rule of thumb is for all-purpose flour and has been around for a long time. Some pastry chefs will substitute even more cornstarch - up to 50% - for specific applications like genoise cake for Roulade.
The exact protein percentage will differ when using that rule of thumb for AP, pastry, and bread flour. But how exact do you need, is a question only you can answer. And how do you know the exact protein percentage anyway?
If you need to replicate pastry flour I'd recommend that substitute with AP. If all you have is bread flour I'd substitute more. If all you have is pastry flour I'd use that as is.