What kind of led would be used for this keyboard
The pads are for WS2812B leds. (SK6812 will also work) (ws2812b datasheet) 
The leds (NeoPixel) in the link you have shared will work with the PCB you have.
Also, just take a quick look at this forum. There the people suggest that if you use the plate that comes with the kit, the leds won't shine through much. You will be better off using some translucent acrylic plates.
You should test the polarity on the pads and align the leds with the help of following pin diagram. Also, just check if you are getting 5v on the pads before soldering on the leds.