Need to add 2nd in the stack and am asking opinions on deep or medium. Keeping the hive in a tighter space and 3rd layer being a honey super? Ready Set …hold on… be nice…and Go!
I keep all my boxes the same size so they are interchangable. Others do otherwise and are not wrong either. Its really personal but I am a lazy beekeeper and like to keep it uniform.
I like to use what we call an 'ideal 'which is slightly smaller than a medium I think. I use them foundationless with wood starter strips to produce cut honeycomb- very good and easy to make in spring when there is a good flow on. Cut combs sells for at least double what you get for honey… I like to use only medium shallow extra boxes when I have a flow hive super as I really want that to fill at leats once per season (preferably multiple times of course).But i am not sure if you are asking about expaniding your brood or adding supers? If it was more brood I would use standard boxes, but I only run single brood hives.
I totally agree with @Rmcpb Why make bee keeping complicated by having different sized frames and boxes, that makes as much sense as having 8 and 10 frame hives !! Keep it simple, it will save time and reduce the number of times you will want to kick your back side.