You're asking all the right questions! These are incredible!
Q: I've heard that the 11th builds on the 7th and 9th and the 13th builds on the 7th, 9th, and 11th . Can you do that or is it just not a good idea?
A: I think you're asking whether or not the lower extension(s) are required to be in the chord. For example, "Am I required to play the 9th in a G11? The answer is, "No, it is allowed, but not required."
Q: At what point do you decide to do a rootless voicing in the right hand, when you have at least 5 different notes to play (9th, 11th, 13th)?
A: If I have a solid bass player, then I try to play more rootless voicings (especially in the left hand, since it is free from playing bass, thus allowing the right hand more freedom).
Q: Should the root always be on the bottom? (I'm guessing it probably should because otherwise it would become a totally different chord, right?)
A: Not necessarily. The root is definitely not required If you have a bass player. You can even get away with it (especially on V7 chords because the 3rd and 7th dictate strongly that it is a V7 chord).
Q: And when you have multiple notes such as in the 13th chord above, how do you decide where to place the notes along the octave?
A: Trial and error, Trial and error, Trial and error, etc. That's the fun of it. There is a great book on II-V7-I voicings called "Stylistic II-V7-I's...", by Luke Gillespie if you can read sheet music.
Q: Finally, can you do other combinations, such as 11 and 9, or 11 and 13?
A: Absolutely. Hmmm...maybe in a future post...