Alright, so far, FIRST unlock all the stages by beating chris and jake's campaign. -- There are so far only THREE unlockable stages for mercs, but there may be more in the future for DLC or for free from RE.Net -- Then beat Ada's campaign.
After you unlock all stages, and beat Ada's campaign, if you haven't played Mercs yet, you'll have Leon, Chris, Jake, and Ada. Beat urban chaos with a B or higher to get Helena. Beat Mining Depths with a B or higher to get Sherry. Beat Steel Beasts with a B or higher to get Piers. To unlock the alternate costumes, beat a stage with any char with A or higher to get their alternate. Once you get the alternates, you'll get Carla Radames and a BSAA agent. That will total out to about 16 characters, and then all the extra RE.NET DLC and soon to be released DLC will be even more. Hope that helped!