The 'adding neoprene' suggestion was actually from the "Manual of Freediving" book, that was recently released. I haven't actually done it myself, although it's not a bad idea if a footpocket isn't exactly 'snug' in certain areas. I imagine some industrial adhesive would do the trick. As for actually getting the neoprene, perhaps purchasing some cheap socks and using them would do the trick? Or...just wear the extra socks perhaps?
Then again, you could bypass all this and just get a good fitting footpocket.... :hmm Just kidding, I know the frustration you're going through, as I really couldn't try too much before I went and bought my first pair. I got lucky with my Sporas, as they fit me pretty well.
If push comes to shove, I'm sure the Scubastore would help you out with finding a good fitting fin. I know when I had footpocket size issues, they sent me a correct fitting pair, no questions asked. So maybe if you emailed them in advance, explaining your situation and your concerns about proper fit, they might be able to help? They say that "wrong size" is a valid reason for a refund...
http://www.scubastore.com/eng/faq.asp
At any rate, good luck! I hope this newbies advice helps you out at least a little. :duh