You can use acrylic over enamel without any problem. Although people have told me that they don't have any problems ignoring this rule, but generally laquer, enamel, acrylic. You can paint acrylic over enamel or laquer without problems, or enamel over laquer. Laquer is more aggressive than enamel or acrylic, and enamel is mire aggressive than acrylic.
As to washing, this link should give you all the info you need:
http://www.swannysmodels.com/Weathering.html
You can also use artists water colors. This link here in the forum will explain that method: http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=11687