If a restaurant (or a host/hostess) brings you crackers with your soup, then etiquette comes down more on the side of abiding by the host's suggestions and/or not hurting their feelings than on the side of whether or not something is declassé. Table manners are always based on what is most polite.
As for crumbling crackers in your soup, well, what is a cracker anyway if not a flat crouton? I say go for it.
What you shouldn't do, is slice your bread with a knife at the table, you are supposed to break off little pieces with your hand. You are also not supposed to keep reaching for the butter dish, you should take an amount of butter onto your bread plate that you feel will be sufficient. This is true of butter served in a ramekin, or of butter pats. I don't follow these rules myself, but was speaking about what etiquette dictates. If you invite me to your house, my elbows are going to be on the table and I will be gnawing at the ribs you serve like a buzzsaw and reaching for the salt in lieu of asking for it to be passed.