I get boxes from work and cut all myself.... even with free boxes I do not feel this has been worth the work even though I still do it. I would definitely recommend ordering boxes because the time and money spent doing your own adds up.
The small boxes will work for babies in good temps when you won't need a heat pad, but if you are new to shipping I recommend avoiding them for if you have to use a heat pad and what not the temps can be a little different than the medium boxes. I haven't had a problem with this in my shipments, but have received shipments where this was a problem. If you want to try it, it will save money, otherwise get the medium sizes like referenced above.
Do not skimp on the thickness of the insulation. I have seen this be a problem too and it is usually because of cost or not being able to find the right thinkness. UPS stores often have styrofoam, home depot has styrofoam and pink insulation foam, and there is the internet, so go the extra mile if you can't find the right insulation on the first trip out......
... I mention the insulation to get to this point: you mention medium size snakes there. The insulation will take up a bit of space. Just be sure you get the right size boxes with this in mind.

Good luck when you do your first shipment!!