Hi.

I need to select related items when i'm creating a new item; not to do it automatically by tag or category, i need to show every item in 6 categories and select 5 from them.

Is that possible?

Thank you.

Anything is possible, but this is not default behavior. This will be quite custom. Start out with the multicategory extension for k2.
By default K2 can relate items by tags, but it selects them for you.