Those Flora/Lilypad things are okay if you want a small project and don't mind paying extra for the convenience of their design. They're usually for weekend hackers that want something premade that they can attach to their clothing for a project. Nothing wrong with that, but it's expensive for what you get. For example, you'd be paying about $1/LED for Lilypad LEDs
compared to as low as $0.01/LED in bulk from China. Adafruit is probably looking to cash in on the success of Lilypad with their own platform.
For a larger project I'd suggest looking into buying the raw parts and building your own stuff. For example, instead of dropping Lilypad branded protoboard for $10 a whop, you can instead buy some cheap protoboards
from DealExtreme or other places. Then you just solder in some eBay LEDs wire them up throughout your clothing etc.
For your specific questions:
- They're not going to heat up unless you're pumping out LOTS of power and light like an LED rope. LEDs don't draw all that much power, and you'll probably be just fine.
- Wearability-wise, you need to make sure that any wearable electronics has a lot of wire slack to it so that when you're moving, and dancing, and biking, the parts won't come popping off. It's one thing to make a suit that you walk around with, it's another to do activities in it. I'd look into strain relief, a sturdy mounting platform on your clothing, and other strategies such as twisting wires together and mounting them along your clothing, etc. Make sure that if you mount electronics that you don't take shortcuts such as neglecting to solder down parts, gluing stuff down, zip-tying everything you can, etc. Things will come loose and it's better to overengineer your stuff than have it fall apart on the playa.
- There are other ways to power your stuff without having to resort to 8x batteries. For example, you can buy some DC-DC converters
that'll take a lower voltage source and covert it into something like 12V (note that you can get these on DealExtreme for good prices too). Still, you're going to need a decent amount of power anyway, so expect to need a decent number of batteries. I'd suggest something like getting some rechargeable RC power packs
so that they're all bundled up nicely and you can charge them up during the day. The batteries in those are also NiMH and so they work better than Alkalines for higher currents.