Sorry, I am new to this server, but can you use both your main skills, and your subclass skills, at the same time?
http://dnwiki.dragon-community.net/index.php/Category:DN_subclass
There are retail subclass and DN subclass.
You can use both skills of base class of your DN subclass'ed char and DN subclass' skills at same time.
You cannot use skills of your main class (class that you choose at char making) when you are playing retail subclass. And cannot use skills of your retail subclass on your main class.
As far as I know, focus attack(warlord's skill) and rapid shot(archer's skill) do not stack (cannot use them at same time). All of self-buffing skills obey general skill stacking policy except for increase power(trooper/berserker skill. it does not stack with might in official but it does on DN).