Downloading and looking through the EVD is perhaps more work than the OP wants to do.
Will the wyrmling be a character on its own? The Draconomicon page 141 has details about having a wyrmling be a spellcaster's familiar, which is probably part of what Cap is getting at.
Are any characters Deepwyrm Drow from Dragon Magic pages 6 and 7? That might tie in and be interesting though the mechanical changes are a bit blah.
As far as the half-dragon goes, the LA for that template is too high and annoying. Perhaps the Draconic template from Draconomicon page 149 would work out better? Only +1 LA and still has plenty of flavor. The
hald-dragon template class could also work since the character isn't required by the rules to finish the progression. Just take the first level so the character has the fluff of being half-dragon while still having actual levels instead of LA. It's a bit of advice for the player and character to make things a little more playable.
As far as the ally goes, a deep dragon would indeed be the easiest. A fang dragon is always chaotic neutral, so alignment stuff might be a problem if indeed the campaign is evil too. Black and deep are always chaotic evil so that would work better. Do you have any other story stuff to go along with all this that might help figure out the setting a bit?