1. Those tubs will be fine - perfect in fact.
2. Flexwatt or heat pad, doesn't matter. They do the same thing so long as either one it on a t-stat.
3. I am floored, shocked, appalled with this seller. There is NO EXCUSE to not send the snake that was ordered without some kind of explanation BEFORE it is shipped. Shipping 2 snakes does NOT make up for that. You really need to post this person's name so we know who to stay away from.
4. I hope your babies are healthy!!
5. Never ever under any circumstances, EVER, use tape or any adhesive inside your snake's enclosure. Once they get stuck on it, they can't get off. If you try to pull it off, you'll pull off scales and skin. There are gruesome pictures here if you find the right threads.
6. Good luck with everything! Ask away with any other questions!