Someone has used some Imperial Assault miniature as a miniature Legion,
Like some medical droid or something?

I know that the scale is not the same,
but there is some miniature that could be worth.

I'm going to use Hera as a stand in for Leia and Ahsoka as a stand in for Luke.

The scales are so close that once you put them down, it will be hard to tell unless you have two of the kind of unit together.

I took a rodian head from IA, as well as the two heroes from the core box: the human (who became a Chiss) and the bothan. All fit perfectly among my troopers.

I dropped Leia, base on base, in and with the added IA base height isn’t as bad as comparing heights directly.

Hera is the only IA model I own, but she seems perfectly in scale with Legion (Hera is canonically pretty short). I plan on using her as a squad leader.

It's also important to remember that people come in all shapes and sizes. I'm 5'10", there's a guy at my work who is 6'10" and a woman who is 4'10" and yet we're all in the same 'scale'. Apparently.

Also, you can 'steal' a bit of height by bulking up a figures base. It's like platform shoes, you don't realise just how much extra height is coming from the base.