You would have much more possibilities if you simply could use one item or items of the same category multiple times!! I don't know much of programming, but to me it sounds very reasonable.
COMMENT BY JANINA:
Sorry, I cannot include this due to technical reasons.
It is difficult to explain... but let's try: There's a certain number of levels (like slots or layers) for items and they have a fixed order. Every level can only show one item at the same time. I use one level for all the hair, that's why a hairstyle disappears when you pick another. It's like: hairstyles = level 30, hair highlight 1 = level 31, hair highlight 2 = level 32,... So it's more complicated for me to add a new highlight than a complete hair style, because I always have to assign a new level. The higher levels are already taken by items. Most items in the stuff category use their own level to combine them freely. But it's impossible to include for every object.