A friend of mine had an NYC based layout and he routinely ran a set of EMD F7 and ALCO FA2 AND his time frame was the 50s. He had an open house for the NYC Historical Society convention one year. One of the visitors told him that was not right for the 50s. The guest told him that until the NYC rebuilt the units after about 15 years of life, they could not MU different mfg units - ie no alcos with EMDS etc. After the stuff was rebuilt in the late 50s and early 60s they rewired them to MU.
This would not apply to GPS and Funits since they are the same internally. One issue would be however that I think a lot of F units did not have MU cables/connections on the nose as delivered. By they 60s these would have been installed again as rebuilds occur.
