Personally I don't think I'd use monster classes for Archons, or the majority of Exemplars (especially the lawful ones). I think it's a decent system for races with a more traditional method of growth/advancement, but something just rings false when you start thinking about making a first level trumpet archon.
The monster classes can work since they are in a way created, then advance to the next rank in the heirarchy of Celestia over time by experience and deeds done. When advanced to a higher station they're normally shed of class levels and HD where appropriate as their essence both mental and physical is altered to fit their new station. A '1st level' hound archon in that case would be one newly promoted to that station perhaps.
This only works for those critters who cannot be born into a specific station (though then they may also be treated as 1st level of that subspecies of exemplar). For example take Nycaloths and Arcanaloths. Both of these types of Greater Yugoloths can true breed and produced more of their type (though these births are considered less suited for their duties than those examples that were promoted up from mezzoloth status through the ranks to their forms).
Baatezu when promoted to a new form likewise shed levels and HD typically to become a less powerful (initially) but more authoritative member of the 'zu heirarchy. A '1st level' example might fit here as well.
Anyway, an alternative you might consider is just let it gain levels in wizard the same as any 1st level human character would. Is there any in-game reason why a Hound Archon would take significantly longer than a human to learn wizardry? No, there isn't, so don't make an artificial one just to preserve a "balance" that can go out the window with enough xp anyway. However, I would suggest that you don't allow the HA to gain hit dice, saving throws, and maybe ability points until he has as many level in wizard as his starting hit dice (so if your HA starts with 6 HD, he won't get the extra d4 from his wizard class until he hits level 7). With feats and skill points, I would say that if you give him all the normal starting feats and skill points of a HA, then he won't gain any more until he reaches 7th level as a wizard. However, if you choose to forfeit all your normal starting feats and/or skill points, then I'd let him gain the usual number of feats and/or skill points for that class with each level.
Personally, I kind of like this option, but I'm sure opinions vary. To me though, the official rules basically just say, "the more hit dice you start off with, the more you suck at being a wizard/cleric/paladin/whatever," for no particular reason.
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I fricking hate the more HD makes you suck setup, though. A werewolf with 2 HD would be a 5th level character, IIRC. Name a 5th level class that wouldn't rock the socks off of a beast with a mere 15 or so average hp (I can't remember their con mod off the top of my head). It is patently ridiculous that all hit dice count equally, too. An outsider hit die is almost as good as a class level, but an animal hit die is only worth a few hp.
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