1/350 People

I mixed 1:450 and 1:500 people on a diorama, but that is a bit less of a difference than 1:350 and 1:400.  Still, I found the mix fine- did not really notice the difference.  One of the sets I used had a number of children, so that helped- you could see a mix of sizes anyway.

A thought- you might want to consider the 1:450 people. I believe I got them from a Japanese importer- there is a new micro size model railroad scale at 1:450 and accessories include people and cars.  These are civilian people, though, but you could probably just repaint adult males.

Using 1:350 scale figures on a 1:400 scale model will have them looking about 12% too tall.

Example - say a figure represents a six foot person (72 inches).    Using  1:350 scale figures on 1:400 model would have them appear to be 81 inches (6'9").     They may appear to be too tall.  While one or two tall figures may be fine - everyone being that tall might not look right

My personal preference is to not mix scales where the difference exceeds 10%.    The difference is just too visually jarring

Test with a few.....  take a few photos in various areas of the ship.....   objectively examine them....  ask your friends....         You might be able to make it work

best of luck with it

hmmmmm not sure myself.