I've also got my LEDs for power light and decoration. I've gone for orange, so it will look like it is toasting while in battle (although I am concerned it may be against the rules as it may look like the robot is constantly on fire? I could always go with red if needed, they weren't exactly expensive).
Orange is fine. The FRA rules specifically state that it has to be a solid, contrasting colour to the rest of the robot (rule 6.4).

Aside from that, I'm trying to get my head around LiPo batteries and what I require. I've seen that 3S is quite common for 2WD on the Argos motors, but with my motors being 20V, I'm not sure if that still applies?
LiPo cells are 3.7v nominal, so 3*3.7 = 11.1v. If your motors are rated for 20V, I'd recommend 5 or 6S (18.5 or 22.2v) LiPo in that case but I would think would still move with 3S, just be underpowered in that case.