Hi there. I'd also make sure you check the tags on the baby clothes and try to use the same fiber content within one quilt so you don't end up with your squares shrinking at different rates in the finished quilt.
Also, I ran accross this under a google search for "Custom Quilt Kits" the other day.
http://www.customquiltkits.com/ I have never used this service myself but it seems worth checking out if you are nervous about cutting things perfectly...although it sounds like you've stocked up on all the right tools to do the job yourself. Practice is of course a great idea- just remember that even if you "ruin" a square- set it aside. You can sew a couple of the bad ones together sometimes and come up with wonderful looking squares. I've seen lots of really fun quilts that have even just one or two funky squares made from different fabrics among basic squares. Kids have a great time finding the odd squares in the quilt.