I would be great to be able to hide selected public rulesets from the dropdown list when creating a new army. As I never play WHFB Dwarves for example I do not need this entry in the list.

For now there are only 3 public rulesets so this is not a problem
but I will make the combobox with Rulesets ordered by number of armies You have already created for it.
So dwarfs will be on the bottom of the list. Will this by good enough?

Later on the Rulesets will be grouped into categories but there have to be more than 3 to start thinking about this

You're right. At the moment, this is no problem. This was more of a suggestion for later developments. Sorting the rulesets by number of armies seems to be a good start.