Can I use 18/2 and 20/2 in the same lamp?
When you state 20/2 and 18/2 are these cables? 2 conductors of fixture wire would be the way I would go high temp rating and easier to fish through the fixture arms. 25w on a 120v circuit would draw less than 1/2 amp 2017 NEC 402.6 states fixture wires shall not be smaller than 18 AWG. Table 402.5 shows 18 awg as 6 amps max.