You can use Acrylic sealers over enamels, and vice versa, as long as you make sure that the paint are completely dry before you apply the sealer.

I usually use an acrylic sealer prior to doing oil washing, because it's harder for the paint thinner to eat through the acrylic sealer.

Here's a pic of my WIP Pershing. I think I've done like...4-5 oil washes on it, in various degrees. Didn't have any problems with the paint thinner eating through the acrylic sealer.