Can I use resistors in series to slow a brushless fan down?

NO, NO, NO! Brushless motors work very differently from brushed motors that can be controlled (albeit ineficiently) with resistors. While brushed fans/motors will take what power they are given and do the most they can with it, a brushless fan/motor is electronically controlled and is not designed to accept power restirictions because of the way they work and some built in safety features (like over load protection). In most cases a brushless fan/motor will work with a series resistor at it's nominal voltage but this will not bring about any speed reduction and use more power. Brushless fans/motors are designed to work over a voltage range so if the voltage drop across the series resistor still leaves enough headroom the brushless fan/motor will work but will draw moe current to compensate. If the supply voltage is forced below the minimum operating voltage depending on the fan it will either just refuse to work or work at a reduced speed but unreliably.