In the official site, in the Mercnaries Reunion section, you see the characters divided like this:
Lost In Nightmares
???????????
???????????
Costume Pack 1
Business (Sheva)
Heavy Metal (Chris)
Desperate Escape
Josh Stone
???????????
Costume Pack 2
Fairy Tale (Sheva)
Warrior (Chris)
That's why the new characters have to be connected to the scenario Lost in Nightmares.
The ??????????? character in Desperate Escape is Excella (she's next to Josh in the Japanese site), so probably she will be the main villian in the scenario.
Barry and Rebecca re not in the game, but the mansion is similar to the one in RE1, so it brings back memories of them (their figurines also have the same clothes and sounds as in REmake). It could be some other characters also present in the scenario, but for now, Barry and 'Becca are my best guesses.
I don't believe Irving, Krauser or HUNK will possibly be in it.
If you buy the game, you'll have all the mercs, but if you buy the DLC, you'll have to buy packs of characters.
Ocidental Website:
http://www.residentevil.com/gold/
Japanese Website:
http://www.capcom.co.jp/bio5/ae/mercenaries.html