I was/am a converted forward playing defence and with agressiveness, it depends on the situation. One on one, don't worry about the puck, keep good body positioning and take the man, if you have a good chance to poke the puck away, by all means, but don't be poking out there wildly hoping to knock the puck away, I'm sure you were already taught this though. Work on you angling, forwards aren't going to do much if you angle them off to the boards and have good position on them. Obviously, agressiveness can lead to penalties if you're not desiplined.
With dump ins and loose pucks in the corner, how agressive you might be may all depend on your partner and where he is on the ice, obviously if an opponent has the puck behind the net, and there's another opponent out front, you're going to vary on how agressive you're going to be if your partner has the man in front of the net taken care of. (or you hope)
I don't know how much of a help this post may of been, but it's difficult to tell someone how aggressive they should play since it depends on a lot of other things. Just keep in mind, don't play outside your limits, and play your game, which should be comfortable to you.