I'm currently in the middle of assigning my military. I want like my marksdwarves to carry a sidearm and a buckler in addition to their crossbow. Will they equip the additional weapons, and when they do, will they use them correctly (i. e. preferring crossbows and switching to their sidearm in hand-to-hand) in combat?
You can assign a weapon to crossbow dwarves and they will carry it, but 95 percent of the time they will just bash with the crossbow. Because of that I have found it better just to train them in hammerskill (the crossbow melee skill) and not equip them with a melee weapon, since they rarely use it. If you do equip them with a melee weapon its advisable that it is a hammer, since that way they aren't splitting exp between two melee weapons.