Can I mix different GAS IT bottle sizes together from 1 fill point.

yes, you can have different size GAS IT bottles connect together on 1 fill point, so they fill together. 

So if your gas locker allows, you can have a 11kg and 6kg for example and because each GAS IT bottle has its own automatic 80% shut off fill valve built in, when you fill up from the fill point that's piped to each bottle, gas will only be replaced in the bottle that needs it.