Greetings!
Hi there Dragongirl! In my own campaign, Dragons have indeed done this, and they have coordinated mass-breeding programmes in order to rapidly create legions of strong, skilled, loyal minions. Just some ideas--
Gold and Silver Dragons seem to mix well with both humans, elves, various half-elementals, and giants. For example:
(1) Mixing with humans provides a great boost to merely human followers.
(2) Elves and dragons mixing provide a range of extra benefits, as well as some common attitudes and goals.
(3) Mixing with cloud or storm giants creates some of the best heavy infantry and shock troops around.
(4) Half-dragon/half-elemental humanoids offer some unusual forces, and these creatures enjoy great thematic ranges of abilities. They aren't necessarily good at infiltrating human society and whatnot, though as special forces that live close to the dragon's lair, they can serve as some very interesting people and special troops. Take a water half-elemental, and combine with silver half dragon template. Combined stats and combined abilities fit very nicely with each other.
Evil dragons have the same options, plus those of the half-fiend template, as well as ogres, trolls, and hags. These offer some unusual abilities, and make excellent servants and shock troops.
Semper Fidelis,
SHARK