Bedford Guy wrote:
I have a 26 year old, rusted to hell, dog ugly, and likely dangerous Federal
Pacific AC disconnect with a non-fused 60A breaker.

What I would like to do is replace this disconnect panel with 100 amp 6
space exterior panel with a 60A breaker for the AC, and use one of the other
spaces for a 20A GFI protected 110 circuit.

Anyone see a problem with my idea?

That circuit is dedicated for the AC condensing unit only. If you look
closely at the wire inside the disconnect you'll probably find a 10-2
with ground. As others have stated a neutral is not present for the
110. If you look in the main service panel you'll also probably find
that it's on a 30 amp breaker. The 60 amp disconnect is a disconnect
only. It's a common installation. In other words, you can't do what
you propose. One solution would be to come out the back of an interior
outlet in that area or, preferably, run a new circuit.