Re: breadboard safety with 6V 3.5A power supply
You shouldn't have any problems. A quick look through some datasheets says the 2.1mm jack will probably be rated for about 5A.
It's harder to get specific information about breadboards.. Wikipedia says they're rated for 5W, which the article interprets as 1A at 5v and 1/3A at 15v. I have a problem with that interpretation though, because power dissipation is measured in terms of the voltage across the component, not in terms of the supply voltage. To make the "1A at 5v equals 5W" thing work, you'd need a 5 ohm resistor. To make "1/3A at 15v = 5W" work, you'd need a 45 ohm resistor.
The actual resistance of a breadboard clip is usually a few thousandths of an ohm. If you send an amp through one, you shouldn't lose more than a few millivolts from one end of the clip to the other and the power dissipation will be a few milliwatts.
I'd say it's a good idea to stay below 2A in a breadboard, not because I expect the breadboard to blow up, but because you need heavier wire when you start passing serious current around. Once you go above a couple amps, the best wire to use won't fit in the holes any more.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.