If I were you, I'd go with a reptilian theme (often overlooked in D&D).
Kobolds as 'pups' or weak minions, Lizardfolk as the next step, with Troglodytes either being a cousin race, or just a specialized tribe, or something. Then, put Yuan-Ti in charge of the whole operation, with a slightly altered flavor to make them less snakey, and more lizard-y. In this case, you have a nice CR progression (1/4--->1--->3--->5--->7) All of the monsters listed easily take class levels to scale for a growing party, and you can put any number of creeps in charge of the mountain horde. Dragons make an obvious choice for a bunch of reptile people, as do the Sarkrith from Fiend Folio. If you want to stick to Yuan-ti, the Anathema from Fiend Folio makes a good "boss."
Maybe make the Yuan-Ti purebloods forcibly "converted" humanoids, and you have the same scary borg-like qualities as the construct army.
Even better, let the mountains be full of Wyverns, and you can eventually have an all-reptile terror from the sky!
Or, if Wyverns are too easy, have them ride mutant dinosaurs, or felldrakes (from MMII, but evil), or even REAL dragons.
You can play this group of foes from level 1-20!!