Keep the music simple. If you are planning on having your cocktail hour just be a meet and greet prior to entering the main ballroom for dinner/dancing , then a background cocktail ensemble should be sufficient. Many venues have an acoustic piano. If they don't the keyboardist can always set his up along with an amplifier. A nice addition to piano is sax. Of course it depends on the scale of your wedding, but it may be appropriate to enlarge your instrumentation to 3 or 4 pieces. These groups can play a range of music. It is always really nice to hear the classic jazz standards bearing in mind that the music is only intended to enhance the time to have a cocktail and finger food. If you are looking for theme music (ie if you are having a beach party or are outside and are looking for something specific) then you can always have a steel drum group....small rock band (playing light) or even something with vocals. A classical group is also nice during cocktails. Ask the string players if they can play pop tunes, show tunes or rock....
In a nutshell, cocktail music for your wedding is really just intended to enhance and be a precursor to the reception.
