Ok, lets see here... (Ah, I see someone beat me to it. )

1- Yes. When not in a rack, you need either clips or bungee cords (better IMO) to secure the middle of the tub.

2- Heat tape is cheaper, but it still adds up fast when talking about multiple enclosures. And you would want to get all that you need in one order since the tape can heat differently in different 'batches'. It NEEDS thermostats, in fact only the UltraTherm pads could be run without thermostats, everything heat related should be controlled.

3- A rack is a space saver, since it goes up instead of out, but it also serves as a way to secure tubs (since the space above each is only 1/8"), give security to the snakes, and regulate temps better (being enclosed). Even I, who hates tubs, built a rack for my snakes since they were taking up too much space otherwise.

4- Stackable tubs work for rodents, but not snakes. Since the heat sources would interfere, you would need to separate them...and for that you might as well have a rack.

5- PAM should be available in most reptile pet stores, but if you google it you should find some good vendors.

6- Aspen and newsprint paper are the best and cheapest substrates for BPs, especially in tubs. Repti-bark is known to mold and harbor bacteria if you don't change it often...and that's just an extra hassle.