All the characters you've found (or bought) will become available once you've completed the 18 story missions (Mount Doom is last). Until then you only have the ones currently moving from story level to story level in the open world. And after Amon Hen (6th story) you'll be even more restricted because the group will split into two, and so you'll lose the character abilities of those not in the present area (you'll be able to flip back and forth between the two groups in their different areas, though). Hope that makes sense.
Some abilities can be replicated, however, by the Mithril Designs you collect in the missions. For example when you replay the Prologue level using Freeplay, you'll come across a Mithril Shockwave Axe in one of the caves that was inaccessible during the initial story run-through (use Gimli to smash the block to get in the cave). After you get the design, take it to the blacksmith in Bree and forge it like you did with the Mithril Trowel. All your characters will then have access to Gimli's Axe-smash maneuver to destroy cracked blocks.
Just play through to the end; you'll get there eventually. :)
Not *entirely* true. Any missions that you can fast travel back to, you can replay in Free Play and use any characters you've purchased, so you can use new characters there. You just can't use them in the Middle-earth world until completing all the story missions.