First things first: Are they dress boots or field boots? Because dress boots are usually only seen in the dressage ring. Field boots will have laces at the ankles and dress boots will have no laces.
As for fit, if it is just the calves that are too big, you should be able to take them to a boot/shoe/leather repair place and they can take them in a little bit.
For cleaning, wipe off grime with a damp cloth, then condition the whole boot, then apply polish to everywhere but the inside of the calf (where your leg touches the horse).
Breeches are going to be a preference thing, but you usually get what you pay for.