Mix and Match - no co-ordination
Well they don't do they! Or at least mine don't. My autistic children keep everything separate, Pokemon in one box, trains in another, Dinosaurs in an old and forgotten one. Their 'play skills' improve with practice, prompting and encouragement, but the idea of giving a Pokemon a ride on a train, or permitting the trains house room in the Pokemon box, is basically a non starter. Even the idea of such a bold step would evoke mass hysteria and panic. Would it be possible to make a 'house' out of Lego for the Pokemon or maybe the trains? Of course not, such cross contamination would be unthinkable! Order must be maintained at all costs. It would constitute a severe blunder on my part to try and jinx the status quo.
I wouldn't mind so much if this was but a thinly disguised 'tidiness' gene. I could also tolerate a little more of the OCD tendencies in this respect, but that's not really the issue here.
In many situations they are incapable of differentiation. They seem 'blind' to the differences or unable to pin point something specific. A cupboard full of food, but they can't 'see' the cereal shelf, the nearest one. Or a barren empty room, with one obvious item of a contrasting colour in the middle of the carpet, but to them it is invisible. But when it comes to toy category contamination, they're as sharp as tacks.
Do I permit this happy state of affairs to continue? Of course not. I take every [many] opportunity to invade their space with a foreign object; a wind-up toy amongst the bricks, a kaleidoscope sneaks into the crayon box, a 'soft' Pokemon mingled in with the 'hard' ones. Do they tolerate the the introduction of such an anomoly? No, they treat it like a virus.
But if you persist, the steady drip, drip, drip, [translation = nag, nag, nag] may eventually take root. For mine I suspect that although part of it is smudging the boundaries of their rigid categories, it is also familiarity as they gradually learn to tolerate a less than perfect world - and we all need a bit of that.