Using mixed media is a great way to experiment and stimulate the creative process. I use acrylic with watercolour on occasions to creat an effect or see where I can go with a piece. One can use pastels with watercolour and acrylic as well. Some artists start a painting with thin layers of acrylic underpainting and then use oil paint for the subsequent layers. It all depends on what suits, but do not be afraid to try new things. The painting below is acrylic with some watercolour washes on top 7x10 ins.