Various Questions...

If they are throughly dry, you should be able to apply enamel over acrylics and vice versa. There are several figure painters out there who reccommend using acrylics as a base coat because enamels, oils and their thinners won't effect them.

Actually the times I have the most problems with a base coat is when I'm using enamels for a base coat, try to use an enamel wash and it does just that, washes off the base coat. And all because I didn't give the base enough time to dry

yeah, but what about acrylic over acrylic, enamal over acrylic, or enamal over enamal? i wouldn't dare use lacquors over anything... i wont make THAT mistake again

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