For a stir fry, the recipe calls for almost all fish sauce but I was thinking of doing 2 tablespoons of fish sauce and 1 of soy sauce? Is this legit?
It's your stir fry, so you can do what you'd like! I think the soy will be fine, but check the sodium content of both ingredients before you proceed. If one is significantly saltier than the other you may need to adjust volumes -- for instance, lower sodium soy will be much lower in salt than fish sauce so you may need to use more than 1 T of the soy.
Also - in general Thai varieties of fish sauce are a bit saltier/stronger flavored than Vietnamese varieties, so i would just taste it and adjust.
I would think so---but I agree with the others to check the sodium content of both. Good luck!
Absolutely. And you should as often as you can.
TF, when i have a question like yours, where i am riffing on a recipe, i will often google something like "thai stirfry recipe" and look through some sample recipes to see if fish sauce and soy sauce are used together in any of them, and, if yes, what other ingredients might be important--- maybe sugar and/or lime juice for instance. That gives me some ideas for improving my own experiments!