Piggy backing power off existing 5v DC circuit?

Just before I blow up an Arduino Uno or myself, I wonder if someone would help me with some simple beginners advice....

I want to build an arduino project inside an existing device (a Super Famicom games console as it happens)
The internal electronics of this are running 5v DC (I am intending to keep well away from the power regulator and not work on it while it's plugged of course)

In my ideal world I'd like to power the arduino from the existing circuit.

From pin outs and schematics online I can see at one place I can get to 5V and ground connection in the Super Famicom.

In theory I think I can connect the GND and 5v connections of the Uno's USB to this circuit and thus power the Uno whenever the main circuit is powered.

Am I right, or am I likely to prevent the main circuit from working if I attach to vcc/gnd OR kill the arduino? Am I providing enough info?

Also, on a teensy board I am using for another project there are solder points on the actual board that correspond to the GND/5v on the usb in plug, so you can use these instead of half a USB lead. Is this also the case for an Uno? My initial googling and looking at pinouts suggests maybe not.

I hope this is a dumb question and I am just reassuring myself.....

Is this also the case for an Uno?

Yes, it is deceptive labels 5V but in fact it is the 5V line.

The success of your project will depend on if your console’s power supply can handle the extra current.

Thanks - so I solder to the pin marked 5v? Or is there another pad/pin somewhere I am missing.

Will try and do some rough calculations on the power draw, but the console itself should not be doing all that much processing the set up I have planned so hopefully should be OK..

Yes just solder to the 5V pin. Some people say that this might damage the input regulator, I have never found this a problem but if you are worried connect the Vin to the 5V as well, just don’t ever connect the power jack to anything other than 5V.