"Моя мама работает в галерее."
Translation:My mom works at the gallery.
Can gallery be also a shopping centre in Russian, as it is in Polish, or only an art gallery?
It was mentioned in an other discussion - some shopping centers may be called "Gallery" or галериа or whatever, but that's the specific name and not the general term for a shopping center.
So no, only an art gallery is correct.
Technically, "in the gallery" means she works inside the building. "At the gallery" can mean either she works inside the building, or on the territory of the gallery (perhaps she does the landscaping and gardening for the gallery, so her work is "outside" the gallery). Pretty nuanced though, you're fine to say either one.
I thought the rule is "Exception: Replace “ия”, it becomes “ии” " is this an exception of an exception