Its a bit of a black art with the Allies loadouts. There are 13 pre-sets, some appear to have just one type of ordinance whilst others have a sequence. if you select a preset (e.g. for Gun 1), its magazine will be loaded with that one untill its full. If you insert an additional pre-set for that same gun then the magazine fills with a pattern of A B A B A B........untill it's full. You have to experiment but my impression is that you can mix to a point depending on whether a preset has multiple ordinance in a sequence or wether it has just one. For me, limiting one or two pre-sets per gun seems to work as well as having a complex sequence and balancing the loadouts so that the loadouts are complementary i.e Gun 1 = Gun 8, etc, etc.
There hasn't been much discussion on loadouts.......I think the philosophy is 'if you have a loadout that works well - keep it to ones self'.........