1. Might be easier to obtain the plastic fittings that are made for interface with plastic water tanks/holding tanks, etc. Sorry I can't tell you where to find those. This place might be able to help you out: http://www.tank-depot.com
Perhaps something like this? http://www.tank-depot.com/product.aspx?id=1758
2.. You could just do a sandwich fitting (inside and out) of something you can find at the home store in PVC using 4200 or 5200 simply as a bedding compound inside and out. Essentially you make a bushing/plate/washer of plastic for the inside and outside that you compress these onto the tank using regular plumbing fittings inside and out. That will work but is more of a hassle and may take up more space than you want. Variations on this theme involve drilling out a hole the right size for your choses plumbing fittings or a fitting with reducer, etc.
Dissimilar plastics (PVC and whatever you've got) probably have dissimilar adhesive properties. We stay away from what we call the "googe-together" type arrangements (glue) and seek out mechanical terminations.