The Sorastro painting video contains a lovely shot of Legion Luke next to Citadel paint bottle, so after making a screencap and using some of my leet MS Paint skills, I came up with this:

Very rough, but shoud give you an idea. My own thoughts being that Legion miniatures will indeed be a couple mm taller and a bit bulkier than Imperial Assault models, but the difference is one that I personally can live with. Notably, the Legion bases seem to be 25mm just like IA, but are quite a bit thicker. I believe that if you could blu tac a plastic disc (like a Renedra 25mm base or two) to the bottom of an IA model, the difference in scale would no longer be noticeable during gameplay. The ability to use IA Stormtroopers et al to pad out Legion armies, even if only for casual matches, means this game just jumped a couple rungs on my hype ladder.

height differences seem mostly due to base size. Look at how much thicker Luke's is than Lando's.

EDIT: Just realized you mentioned that in your second sentence. Hooray for reading!

Just now, Rocmistro said:

height differences seem mostly due to base size. Look at how much thicker Luke's is than Lando's.

Luke is a bit more hunched over than Lando so I believe him to be a couple mm taller, pose notwithstandind, but yes, I believe most of the height difference is in the base, so if you have some spare wargaming bases and a bit of blu tac, happy days.

With this comparison, with the bases changed to the same size i think you will hardly notice any difference in scale and should be able to mix the two together.

I'm a bit slow in the head today, I just figured out you shouldn't need blu tac at all. As seen in Sorastro's video, Legion miniatures do not come with an integrated base and are glued to their plastic base. Renedra makes plastic bases that are dirt cheap and are about a half to 2/3 the height of IA bases, so you could just base Legion on those instead of the bases they come with and you'd be all set.

http://www.renedra.co.uk/product.php?product=135

Forget Imperial Assault, how does the scale compare to Lego Minifigures?

Are you suggesting proxy-ing lego minifigs? Now that is something I'd like to see.