Adding Under Cabinet LED Strip with Direct Power

I plan to add under cabinet RBG LED Strips. I have a 1-gang light switch box that controls my pendant lights. I plan to convert that to a 2-gang box and tap into that existing power to add another light switch for the under-cabinet lights.

I plan to have 1 switch that controls all the-under cabinet lights. There is also a hood-vent between cabinets I plan to go up-and-over through the cabinet.

I feel pretty confident in this project, but have a few questions.

1. Is there any reason I wouldn’t be able to tap into the existing 1-gang switch as a power source? My buddy told me that I have to use an existing outlet, not an existing switch, but I didn’t think he was right.

2. This is on a 20amp circuit with 12/3 wiring, not 14. So, I’ll have to use more 12/3 wiring. Is there any reason that would be an issue with LED strips?

3. I am not sure what type of wiring to use between the counters where the hood-vent creates a gap. There are cabinets above the hood-vent I plan to run the LED Light wiring through to connect LED strips, I am just not sure what type of wiring I need to buy for that. I haven’t been able to find a clear answer. Does anyone know?

Thanks!