I dismissed my entire Militia army because I just unlocked Troopers but I keep finding random amounts of militia in my army info and I keep having to dismiss them. I'm losing might every time I do it and I'm not training any of them
Several quests reward you with troops of different quality. Other ways to gain random tiers of troops are through the campaign.
The might you are losing from dismissing the unwanted troops are only from those troops. I wouldn't worry about it.
Dragon Tamer of DATC