Xochimilco is fairly quiet in the mornings and on weekdays, but like most places, it becomes very crowded on the weekends. We visited in the late morning on a Friday and it was absolutely serene and perfect. Reserving a trajinera is not necessary and you can negotiate with the trajineras upon your arrival. Trajineras should cost around 350 pesos or ~$19 per hour, but if you book for multiple hours, you may get a better deal. Remember, everything in Xochimilco is up for negotiation!
There are a mixture of tourists and locals on the canals and everyone is welcome. The weekends tend to be filled with more locals who come to Xochimilco to celebrate birthdays and other special events.