Mmm... I think you're overcatering a bit actually. If it was me, I'd stick to two meat and one veggie option. With burgers, people tend to eat fewer (IME) but with sausages it's worth doing a few extra as everyone only takes one or two, but then they nibble on them. especially if they're good sausages (and please, please please, if this is your first bbq ,remember to be careful not to accidentally over cook them - it's easy to do).
I'd also serve the bread rolls seperately so that the vegetarian can have some and those having burgers/sausages just pick them up as they need them rather than you "packaging" them up as it were.
I also think the garlic bread is a good suggestion and would be temptd to replace either the potato or pasta salad with that instead? (if making your own garlic bread, a squeeze of lemon in the garlic butter can really lift it to a whole new level).