Frequency does not affect wire gauge.

This is approximately true at the wire sizes and the power-line frequencies that we are talking about. However, the skin effect concentrates the current around the outside of the wiring increasingly with frequency, requiring a larger diameter wire to carry the same current.

What it actually does do is make transformers smaller/more efficient

That is true. It also makes motors smaller.

at the cost of magnetic interference.

Not really. Wire shielding is not very effective at such high currents and low frequencies - whether 50 Hz or 400 Hz - and so we protect against radiated emissions from power feeders by physical wire separation.

Wiring has parasitic inductance and capacitance that cause reactive currents to flow - increasing power losses with increasing frequency. The distances are relatively short in aircraft, so we don't care much about that. We care more about the weight reduction that we get with higher frequency.

In contrast, large utility-scale electrical grids have enormous lengths of wiring, so they care very much about power line losses and less about weight. That is why they use 50 or 60 Hz.