Weasipoint
, your thoughts and the replies above pretty much cover the issues affecting your knots.
As far as cinching goes, with snake knots it's best to attend to cinching each one as you go. Get the first one as good as you can--it's harder to get one right without another one next to it--then, with the second, get it pulled up as close as you can to the first, put the thumbnail of one hand against the high point of one bend and push that toward the first knot while pulling on the opposite side free end. Do the same with the other bend of that second knot while again pulling the opposite side free end. Go back and forth until you get it cinched and balanced then go back to the first knot and do the same from the knife end to cinch the first up against the second, then proceed to a third and repeat what you did for the second.
If you're tying snakes using the loosen-and-slip-through method, I would suggest going back to
Post 825, Page 42 and trying that technique. It's much easier to get you knots cinched up and uniform that way. If you're going to intersperse a bead between two snakes, choose one with a hole big enough to take both cord thicknesses--going through with just one will never look good or allow especially the trailing knot to look right.
As far as thick cord goes, 750
is tougher to tie and cinch up properly than 550, but it can be done well once you've mastered your knots. As you'd mentioned the cord you used was stiff, I'd advise always using cord that has both a nylon core and nylon jacket. Poly jacketed cord is harder to tie and doesn't lay nicely IMO. Make sure of what you're getting if you buy online. If you buy in person, make sure the cord is soft and lays slackly.
It's harder to re-use cord that's already been tied up, but you can usually loosen and relax good cord by working it both longitudinally and at right angles between you fingers. If it's crap, it's crap and throw it out. Good nylon/nylon cord is cheap enough at around 5-6 bucks for 50' or 8-9 for 100, especially as there are sellers who will ship for free.
Good luck with your knots--keep at it and you'll start getting nice results.