A Namespaces is not a Category
If pages are related assign them to categories or turn them into subpages.
Namespaces have a special purpose in Mediawiki that goes beyond dumping a bunch of (apparently) related pages inside in order to denote their relation. The syntax for namespaces is the same as for Interwiki links and careless use of each can break thing easily. Namespaces exists to distinguish among content types like aricles, discussions, media files, templates. They can be related, but they are not the same species. Articles can be separated into different namespaces to allow for public (Main:) and internal (Project:) documentation.
Most namespaces are not searchable by visitors per default, which might be what you want, but this doesn't prevent them from appearing, e.g. by transcluding pages across different namespaces. Navigation between equally named pages located in different namespaces is not available by default. It's possible and can become a very powerful feature, but requires custom made navigation templates. Automatic linking between similar pages located in different namespaces only happens with very few built-it namespaces, i.e. Talk and Main.