If you are wearing a tiara made from flowers, guard against making it seem like a wreath around your head. It might look like a crown of thorns. The Juliet cap is particularly flattering to some who is a little over-weight or is short and looks best worn with a short veil. Try many combinations to find out what is best for you. You might find that you look especially good in a mantilla even if you aren't tall and stately.
Most headpieces come with a veil.
The long cathedral veil is generally worn at a very formal wedding and often serves as the train on the dress. If you plan to dance at your wedding, you may want to order a veil that can be snapped off at the headpiece. Silk is appropriate with every fabric. Satin would work well with a winter fabric. Linen is good for a summer fabric.