Yes you can intermix the same types of paint, example; oil with oil. But, I don't suggest it. It is impossible to predict the actual out come. Will the new paint be worth using or will it turn into a real mess? Also, if you need more you will have to purchase the same types and brands then mix them in the same ratio.
Galvanized metal can be successfully painted once it is properly primed. You can use a good standard house paint. New galvanized metal should be wiped with white vinegar to clean and etch its surface.