Firstly, great job! I'm looking forward to another 'expansion' for my beloved game. Especially in the light of it's newer 'iterations'...
Lamassu is shaping up to be my favorite one from the bunch. I love mythological units and this one really looks like it would fit into the game. If you ask me, design-wise I would choose I->II, but the upgraded form would look even better with some more 'ornamets', cause it doesn't stand out that much from the non-ugraded form. I think that Assyrian-esque hat from your earlier designs is a good choice. Color-wise it should be kept in golden-red scheme, whithout black. Oh and keep the horns downwards, it looks better that way.
About the other units, he're my opinion if I may:
Lvl 1 -> While I love a werecat idea for a unit, unfortunately I dislike your design of it. It looks a bit misshapen and underwhelming, even for a lvl 1 units. I would prefer this type of design, but It's your vision so I won't criticize your choice...
Lvl 2 -> ...whereas this one I gotta criticize though. This bee/wasp unit seems completly out of place (even if it was present in M&M games), and I really dislike either the idea behind it, and it's design. Maybe a Pazuzu would work better here? Seeing how there's already a Assyrian Lamassu in the line-up. But seriously, drop this one...
Lvl 3 -> Snake-man. Now this one is just awesome! There are many snake-like units in the game already, but this one should be kept simply for it's great design
Lvl 4 -> The Sand Golem is ok I guess. But ok at most. The game seems to have quite a few Genie/Efreet humanoid-ish units so another one is not that much exciting. Even if it's design is interesting.
Lvl 5 -> Hmm, another mage-like unit? Not that much exciting but maybe it will have some nice abilities?
Lvl 6 -> Now, the Scorpion here is another awesome unit. In terms of sheer awesomness it's falls short only to the Lamassu. I would never guessed it as a tier 6 unit, but I don't mind. Really cool.
Additionaly, adapting the Megadragon into Heroes 3 is another fantastic idea. One of my favourites from Heroes 4, and more dragons are always welcome. If I could give you a small sugestion though, change his eyes. Notice how all dragons in H3 have more of this "feral" look in their eyes, probably because they have no pupils...