Hi Adlemsi,
I usually cut my oncidium spikes after the blooms are finished falling off...just as you thought. I cut them down to about 1 or 2 inches from the bulb, being careful not to nick the bulb with my pruner.
As for repotting, I grow mine in the same potting medium I grow my cattleyas in, and I make sure that the plant is centered in the new pot with about an inch around on all sides. After repotting I make sure that the plant is secured in its new home using a potting clip. I usually repot in the spring (so long as there are no actively growing spikes), and only when the plant starts to grow new bulbs over the sides of the pot it currently grows in.
I hope this is helpful to you.