Just to echo what has been said, all 3 sponson weps are fine as fiction fit since they are vedge shaped aslong as you dont remove the flash on the vedge. Add a layer of undercoat on the whole wep and the fit becomes even tighter.
Turret, yes i can confirm, magnetising the upper and lower turret parts is all you need to do, in order to swap weps easily, i did it on mine and works nicely.
one 5mm mag has more then enugh strenght to keep the two halfs together, but you can allso use 2 3mm if you want to feel extra secure.