I can see both sides with proxies, I don't believe things should be GW models only, if other companies make models you like more, go for it. It is better I think if they are easy to explain, and if they are roughly the same size, different bases for example could make the game harder/easier. Personally I have seen some great dreadknight models somewhere I intend to use for blood knights, if I can find them again. On the other hand I did once play a chaos player whose knights were eldar jetbikes, flamers were assault marines, and a dreadnought was a greater daemon. The guy wasn't a new player, had used these proxies for 18 months and just didn't want to bother buying any models. But I think what bothered me about it was his attitude as much as anything, if he felt he was losing, he would simply remove all his models from the table "the army has committed ritual suicide". Annoying.
And Aldramelech, fix the rest of the post you edited.