Use the POR15 primer... I tried painting a 72 Charger R/T with POR15 in the trunk. Customer wanted the trunk floor and wheel wells painted red and nice to match the car. I put the etch primer on it (PPG not POR15) and it didn't stick. I told my (old) boss that we needed to use POR15 but he was too cheap of a bastard to buy it. Well he found out the hard way and spent more money in the end having to buy the correct primer.
I have heard of people getting away with non POR15 primer but I would use their primer and not take a chance. Now I was reading on the Lizard Skin about how it seals off moisture, etc (like a bed liner like Line X, Ryno, etc) so you may not need POR15 under it now that I think of it. Rust only spreads when air and moisture gets to it and that is why bed liners keep anything from rusting further since it's completely sealed layer. Now if there are parts you can't get to with the Lizard Skin then I guess you might as well go with POR15 under it...
Either way, hope this helps some and good luck. Whatever you do, dont skimp anywhere. Your already this far in with this much time and money. Skipping steps and going cheap now will cost you in the end. I might be young (only 21) but I've learned my lesson the hard way and it's taught me well.