I'll address a couple things, heroes all come from Nightfall and EotN, so expecting to use them in Prophecies requires getting to Lions Arch, and heading to those campaigns for a bit to unlock them.
There is only 3 heroes of each profession (with some exceptions).
For mesmers theres two, Norgu and Gwen, and you can swap Razah to be a 3rd (he is able to change primary profession, the *only* hero that can). So 3 possible mesmer heros.
The mesmerway hero team *absolutely requires* multiple mesmer mercenaries... how many depends on if you want to beat all of NF to unlock Razah.
Also, Merc registrar is in LA in Prophecies so that's where you'd gain access to any mercs you do buy and make.
(Reminder: if you don't buy the merc 8 pack first shot, you're stuck buying the lower amounts for more overall $ to get to 8. You can't buy just a single one to test then add an 8 pack key to your account)