[quote=@Kindos]
One question about the assassin, is it the conventional assassin you think of?
Like being hidden in the world of shadows and really stealthy
[/quote]

Like Dark elemental powers (absence of light, not evil)?  Cloak yourself in shadows or be invisible and attack people while they can't see you?  That would work.

Basically, most versions of these classes will be fine, as long as it fits the description (in your case "Assassin") and isn't too OP compared to the other characters.

For example, it doesn't matter if the Dark Warlock uses mind control, fire, necromancy, or summons lesser demons, as long as his skills still fit the definition of a Dark Warlock.  For example, a character heavy on melee and light on magic wouldn't fit this description, though it could be a warrior or, with the correct magic, an Assassin or maybe a Spy.