One of my personal rules is no switching pistols in the middle of an engagement.
From what I understand, switching pistols involved removing the eels from the tubes and physically changing the pistols, not just flicking a switch on the TDC. During a reload I may switch if I think it necessary.
That being said,
I mostly use impacts until later in the war. If I do use mags, the sea needs to be calm-they tend to premature if the seas are rough.
Mags work pretty well against capital ships-the armor belt on them was designed to minimize torpedo damage.