Cutting a slot and running a GPU @1x would not be advisable! Now cutting a 4x slot would be a slightly different story.
Nonsense, a PCI-E x1 slot will be ample for what us miners require. Cutting the actual mobo might be tricky, but if you're careful it should be fine. I wouldn't recommend cutting the actual mobo, but as it's old and you seem keen I don't see the harm in it.
For some advice on how to cut the extender check out this guys blog:http://blog.zorinaq.com/?e=53
Crossfire isn't needed for mining and is slower in most cases, you'll have to run a miner for each GPU.