For the first question, yes entirely acceptable although there's also a 'drive' as you put it to make things consistent even if only internally consistent. For example, the old Eldar Warp Hunter (
http://eldar.arhicks.co.uk/miniatures/o ... er_1_1.JPG) doesn't exist any more. So if you're using the model counting as say a fire prism, then it's best not to use other models for fire prisms too. Having said all this, it's up to your opponent what he puts up with! My knightworld army is a mix of Battletech and Gundam models for which I just say that the former are all Wardens and the latter are all Knight Paladins for instance. Easy to remember during the game.
As for base sizes it looks a bit messy on the table, but acceptable within the rules.