The basic answer to your question is another question. What fabrication code/specification and you working to? If you are fabricating components for a client, there must be some document that they have issued to you that provides guidance. If you are working to virtually any fabrication code, e.g., AWS D1.1, ASME/ANSI B31.3, etc. you will be requried to qualify a WPS and your welders will need to be qualified on that WPS.
js55 is correct, in that welding carbon/mild steel to stainless steel is done everyday, normally with success, if those doing the welding know what they are doing. The use of a "specialty electrode" like the Magna 303 is a waste of money, in that it has no benefit over a normal AWS classification electrode, such as E309-16 or E312-16. Standard practice for the maintance welding companies is to "take it from the big box, print your special name on it, put it in the little box and sell it for 2-5 times the normal price". Good business if you can find someone to fall for it.
A good quality electrode from a quality supplier (Sandvik, Esab, Avesta or others) will do everything you need in welding stainless to carbon/mild/low alloy steels.