Yeah, the rules for monstrous creatures and vehicles say that the 25% obscured rule takes precedence over area terrain.
It seems pretty clear to me that providing your models are big enough, you can take cover from them, just make sure they are fat people to get the most out of your models. I never thought about this, I'll be sure to make the most of it in future.
As for the forest question, I would just make sure that you and your opponent discuss the definition of that rule in advance. If you take it on face value it says something like "if the shooting unit has to trace its line of sight through a forest...". Which to me suggests that if its behind the forest, or in it... it grants a cover save to the target. but it does seem a little odd if they are in the forest but less than half an inch away from the boundary, shooting out. Maybe it would be worth using the old 2", or some variation on it rule as a house rule.