Electrical – replace the aluminum cable of a single circuit with romex

Most likely.

As long as the fixture has a UL (or equivalent) stamp/sticker, there is no problem.

Fixture wires, and the wiring inside devices is specifically designed to carry the rated loads. There are different rules applied in this situation, since the load is known and static.

According to the National Electrical Code 2014 402.5, the allowable ampacity of 18 AWG fixture wire is 6 amperes. Your fixture will only draw 2.5 amperes.

I = P/E = 300 Watts / 120 Volts = 2.5 Amperes

If the manufacturer could use smaller wires, they probably would. However, NEC says 18 AWG is the smallest allowed for fixture wires.

One of the reasons light socket adapters can be dangerous, is that they can allow more current to be drawn than was intended by the fixture manufacturer.

First thing you'll have to do is determine where the wire from the switch is going. I'm guessing that all those lights,fan, and receptacle are just daisy-chained with the switch interrupting the neutral inside a fixture box or separate junction box. (Why, why, why?) If you make the existing switch control only the ceiling fan, you'll of course, need to decide where you want to add a switch or switches to control the others. Additional wiring may involve crawling in the attic if there is one, drilling top plates, and dropping wire to new switch locations. That's the best I can offer given the vagueness of the problem as described. I will add that, if necessary, you could probably rent a toner and probe to trace wiring (with circuit breaker turned off). Or, more practical, find the suspected other end of the switch wire, disconnect it, and gently wire nut the pair together and then check with a multimeter for continuity at the other end to confirm that's your wire. HTH