Aren't all minis just proxys ? because 8 foot tall genetically modified super troopers are difficult to come by in large quantities ?
Seriously, that exactly what i'm doing. For me its time and patience as much as money. If i had to paint 50 more guard stands for each variation i'd go crazy !

Same goes (eventually for SM and Eldar. In the future I'm trying to be clever. For example, my IG is krieg grey, i am planning to paint any more IG tanks in grey/karki cammo. Think they will look fine together as Steel Legion/Cadians, but i could also split them eventually to have a Ulani/Minervans
and Krieg/Barren options to face each other.
As long as its clear whats what i cant see any reasonable person having a problem with it - if they did i'd suggest not playing them !

But maybe things are tighter in tourneys ?