I would certainly order a vegetarian meal, but if there is something on the menu that suits too then there is nothing to stop you from asking if there is one available instead of the veg meal provided.
Originally Posted by
Genius1
I would pre-order a vegetarian meal, as if for some reason there is no vegetarian option on the menu you may end up going hungry as IIRC cabin crew aren't allowed to serve meals from a lower class.
As for mixing products between cabin classes; this is definitely frowned upon, however I can't see any problem with a passenger in First having a meal from any other cabin if there is one available. I will endeavour to provide any catering for my guests in First that they would like, as long as the other cabins have had their entitlement.