You may find that watercolour brushes could be too soft for acrylics, but if you are using them like watercolours then they would be ok. Just make sure you dont let the acrylic dry on the brushes as it is very difficult to get off. The small watercolour brushes may be ok if you want fine detail.
You can use oil brushes if using acrylic thicker. The brushes from the bargain stores will be ok for you to start out with, but you may find that you want to change to better brushes as you go. Some synthetic brushes are also ideal for acrylics. I have some System 3 acrylic brushes, and also some Royal soft grip ones which are very inexpensive.
I do find that brushes are a matter of personal choice, so just have a go and see how you find it.
Hope this is of help to you.