a) Do you thing it will be better to put a namespace or use of category to setup photo web page?
A Category of images is basically a gallery already. I would only use namespaces if you need two pages with the same name or if you content is different like a user manual vs. articles; not differences like Van Gogh vs. Monet.
b) Is there a way to have a different toolbox setted up, when using specific namespace or category
Yes. Only way I can think of at the moment involves you knowing some php. But there might be a couple extensions that change skin depending on what namespace you are in.
c) I know how to setup a gallery, but is there a way to setup a primary article displaying only the first image of the gallery?
Just make a regular article and put your initial image in it. Wouldn't that work? Depending on your version of MediaWiki there is even a built in feature to allow you to click through an image to some thing other than it's image description page.
Good luck! -Courtney
PS. You may want to send these kind of questions to the MediaWiki-l list.