This should help:
Sexing Chinese Hamsters - Vectis Hamstery
Male Chinese have ehem....obvious ends!
The females are not as prominent. If you are sure it is Chinese hamsters you have, this guide will tell you what you have in terms of males/females. But if they have always lived together in the shop, I imagine they are all the same gender.
Make sure they are all getting along even when you remove the shelf from their Duna. If any behavioural changes are noticed like stand-offs or any continual chasing or attempts to bite the others, then I would definitely separate them if you see this happening as Chinese hamsters can fight usually when you least expect it. The Duna will only have one gap for one water bottle, so when keeping a group like this, it`s better to have two water bottles and scatter feed. More than one wheel is also a good idea as they might squabble over just one wheel.
It`s a fine balance really. If they are happy with what they have, then introduce other things slowly in case adding too many new things causes bickering.
Let us know how they are doing.