First, 250 watts into 6 ohms is about right. It's not going to be quite that high but it will be close.

Listening to two channels will mean that there is incrementally greater power available. Since each channel has its own tap off the transformers, there isn't much that any channel can do to drag down wattage available to any of the other channels.

My mains are 4 ohm, center and surrounds are 8. I can't tell any difference in the mains when driving the center and surrounds.