For 12-13W and a 12V battery, that equates to roughly 1A, so you could do with at least 2A diodes. This means anything above this is okay, 6A is maybe a little overkill but will work.
Follow the diagram in the thread and you should get dim/fully lit on the turn signal, the 470Ω resistor should be a 0.5W or above.
The 470Ω determines how dim the light is when the blinker is off, and will only pass around 25mA maximum (see Ohm's Law). The diode does pass the fully lit current of around 12W / 12V = 1A, so it ideally needs to be rated for at least double.
First, a bit of nomenclature. What you have are not transformers. They are DC power supplies. Sorry, but I'm picky about terms.
Second, power supplies like you are buying are usually about 90% efficient, so you'll need about 850/.9, or 950 watts.
Third, your power supplies take 120 volts in, so you will need 120 volt AC power for them. It's true that you can get this from 208 3-phase, but at your level of knowledge that's not a good idea.
So. Will 20 amps at 120 volts do what you want? The power available is 120 x 20, or 2400 watts, more than twice what you need.
Sounds good to me.
Connecting your supplies to AC is best done by connecting a standard power cord to the input of each supply. Start by looking at the data sheet which comes with the supplies. Then take one of your supplies to an electrician, and ask him to show you how to connect it. You should be able to do the others by yourself, but make sure you pay attention to the color coding on the wires. Finally, buy an AC power strip, plug it into the wall socket the show people point you at, then plug the power supplies into the strip. You should be all set, and the power strip will have a master switch which will allow you to turn all your boxes on or off.
Best Answer
Just because they are in the same consumer/retail/commercial package does not mean they are from the same production run, or same bin. They are not guaranteed to have the same diode properties for forward voltage or current or brightness.
You can attempt to match them for current and brightness, but the simplest solution is to use one resistor per parallel circuit. Resistors are cheap and small.
The typical efficient setup for white leds at full brightness 20mA @ 12 Volts is three leds in series with one resistor. Repeat for additional strings of three. You could do two per resistor if you adjust the resistor value.