That is insane, Draake. I'm very sorry you had to make that kind of experience. It is... tiring. I mean, what do these people think?
How to deal with those situations: you did exactly the right thing - you won! It's the best you can do. People who act unsportsman-like obviously care little for the fun of gaming and sharing it with their opponent: they want to win. And if you take that from them, you have stripped them of their sole pleasure, which serves them just right.
Apart from that I think that communication is the most important thing ever. These people should be told what you didn't like, maybe they act the way they do only because nobody ever told them it's wrong. But in the midst of the game there is much tension, and starting a discussion is almost never productive. You should make a remark that there is need for discussion after the game, and then go on playing. So when you go 'hey, how come your dreadnought assaults from the drop pod?' and he goes 'it's supposed to be a Lucius-pattern drop pod, the rules say you can assault from it', then I think it's best to say something like 'I can't consent with that. We'll have to talk about that after the game'. Then you can talk it over when tempers are cooled, when it doesn't disturb the game, and when the opponent doesn't suspect you're trying to take advantage of the rules. It is important to note the need for discussion, though, because if you lose and discuss an issue after the game the opponent may think you are just a bad loser in search of a scapegoat.
On proxying: my gaming group proxies a lot.
But first off -like the majority here- both players go through each unit in their list before we even roll for mission. You get introduced to your opponent's army in complete detail. This is one of the most important "rituals" we perform when playing our games.
And secondly we would not allow an all-infantry squad to be a proxy for an assault squad. It is perfectly okay for us to take two or three assault marines and fill the rest of the squad with infantry models if you are in need of them, but there has to be something in the unit that hints at its nature, because otherwise in the heat of the battle you could easily forget about it. It is okay within our group to use a Necron heavy destroyer as another normal destroyer in a destroyer squad, or using Pariah models as additional warriors in a warrior squad. But infantry marines as assault marines without at least a handful of jump packs in there is just insane.
On Forgeworld rules: we wanted to include them in our campaign and everyone went over the models and possibilities offered and conceived army lists. And when we met again next time we all agreed without any discussion that the plan to include the rules was stupid and that we should ban Imperial Armour from our games.
The point being that some Forgeworld models are just awesome models, not being particularly effective, some even being less effective than normal models. But there still is some units that just break game balance. The Lucius-pattern or the Deathstorm drop pod are two of those in our opinion. They are just too cheap for what they do...
And I totally agree that if you decide to use them you should at least make sure that your opponent knows what your units (can) do before setting up, and that he is okay with it and with you using those models and rules...
