RE: your 1st question: The secret to DIY is repeatability! Whatever you do consistently repeat doing it - keep notes on each and every recipe and/or use an e-juice calculator like E-Juice-Me-Up... As far as the amount left in the needle - rinse out you syringe between each flavor... Not that the miniscule amount retained in the needle would affect the juice - just good lab procedure...
RE: your 2nd question: Yes, you'll need larger needles for VG... Get a small range of them - they're cheap...
I figured it was a minescule amount, just that with 3mL sizes maybe making a bigger impact. I've seen 14, 16, and 18 gauge needles at Wizard labs, should I just get a few of each?
Throw those syringes away and get a scale. Seriously.
I've seen a lot of people saying this is the way to go, and I will do that once I am making larger batches. For now, I'll stick to volume measurements. I can only imagine it would be a pain weighing everything out for a 10mL bottle. Of course, a 120mL batch would be a pain filling even with a 20mL syringe, so thats probably the point I'd do it by weight, lol.
Make sure to use several coats of clear nail polish on your syringes so as to keep the markings from wearing off. It really works well. A good hot water rinse keeps them clean and if that doesn't satisfy you an alcohol flush will do to sterilize them.
Ah nice idea, thanks. I make sure to keep my syringes and needles cleaned AND sterilized with alcohol

I'm assuming start 70% rubbing alcohol should suffice my needs?
If you suck a little air into the syringe before drawing up the liquid (instead of starting with the plunger all the way down), the air in the syringe will help push everything out.
Also, you can never have enough bottles - well, I bought 24, and I'm only using half of them at the moment, so ymmv. Search Amazon, or your favorite supplier for '2 Dram glass vial' - often cheaper than plastic bottles, I don't get nervous rinsing and reusing, and the hold about 6 ml, perfect for a test batch.
Mix on, and take good notes.
Another nice idea, I usually do this so the amount that's left in the syringe is probably 1/20mL if that. That satisfies me that the amount will make no difference on my batches. Thanks!
So I'm just going to go with Wizard Labs for all my syringes, needles and bottle needs. They have some glass bottles there, too, maybe I'll try some, but I really like the plastic bottles. Thanks for the help guys!
Oh one more question! I am mixing with 100mg/mL nic. If I'm understanding the concept correctly, this is 10% nic and 90% PG (the type I picked up is PG based not VG based), right? Also, I'm going to be mixing it indoors somewhere, probably my garage. It's like 10 degrees out of late so I don't really wanna crack the window if I don't have to, would a small fan to keep the area ventilated suffice enough to keep me from any chance of inhaling too much of the nicotine when I crack it? I also have a son, almost 2 years old, he doesn't play in the garage but his room is in close proximity to the garage (the doors within 20 feet). As long as his doors closed, he should be contained from the Nic?