To get the solder out of the front fan header use a solder wick.
The gauge of wire to use for your fans and leds depends on current going through the wire.
For leds, 30 gauge is fine.
For the fan, it depends on how much current it is rated at. Should say on the fan. It may say how many watts it is, but to find out current, take watts/volts.
Then use this chart
to determine minimum gauge.
Most likely 30 would be fine but with multiple fans and led's, 28 would be best.