Welcome! Here's my attempt at answering you:
1. Yes, the coupler can attach an initiate to a Dominix or Aeon. They're compatible from what I've read. However, I personally prefer the sabers of my staff to be very similar to create a more balanced staff.
2. The standard length of blades for the staves is 32". This means each blade is 32", though they're down a few inches in the hilt so it's more like 29" each. This adds up to 58" plus another two feet or so for the hilts and you get a grand total of about 82". My friend Artorius and I are of rather different heights, and we both find this to be an appropriate sized weapon, but if you are looking for a shorter staff there are two additional options: the Phantom Initiate (two Initiates together as a staff) and the 24" Initiate blades they sell as an alternate blade option.
3. I wouldn't assume that the quick ship will be sent first - if you order the stuff all together it's gonna ship together. The price they give you for shipping is the cost assuming all products are shipped together, hence why it doesn't go up much for additional sabers added. You could email them at
shipping@ultrasabers.com and ask, or just put it in your order comments and see what happens. If you really want the quick ship to be sent out when it's done, I would make two separate orders.
4. I would NOT assume any of the sabers are waterproof - that video was made when the sabers were primarily PVC hilts, which made them effectively waterproof. Now they're made of various metals, and the sound sabers have all sorts of delicate electronics that you would surely not want to put in water. If you want a cheap saber you can use in the water, maybe go for a PVC grab bag or something.
5. I'm not sure why the UEHG blades aren't an option for Initiates. It should be compatible, all US blades have the same diameter in order to be interchangeable, so I would try emailing them or just ordering the heavy grade and asking in your order comments for an Ultraedge heavy grade instead.
Whew... hope that helped. Good luck!