Battlewagon bits (linked in the shopping thread) is your friend. Buy that second ML
It boils down to this: If youlve paid the points for it, it HAS to be on the model
It's a one-way street though. Just because you have it on a model doesn't nessecarily mean it needs to be on his sheet
Grenades and skills seemt to be the exception/gray area in the WYSIWYG rules. Some people will argue that everythign needs to be there, other people cite a rule form an older edition (I believe) that says grenades ad small stuff need not be reresented(hell, a 5 man assault squad sprue only comes with three meltabombs)
Most people will go for a compromise. If someone (preferably someone wihtout access to wargear) in a squad has an 'everybody must take' upgrade like grenades, modeled on the mini then you can assume the whole squad has them.
In any event, whether the model has things on it that aren't paid for, or if you're in a friendly game where WYSIWYG isn't a huge priority, or you have skills or other unmodeled things, ALWAYS INFORM YOUR OPPONENT