I was bleeding the baseboard heaters (and I also want to know if it is normal for only one room out of six w/bb heating, to have bleed valves), but now I cannot get one of the screws to stop water from leaking no matter how tightly I screw on. Can you over tighten these things? I have a feeling the heaters haven't been bled in 3-4 years. Should I just keep trying to tighten? Do I neednew gaskets? Should I get a tech asap? How much water can leak out before it gets dangerous?

I dont believe any of the systems have gaskets in the bleeders. you might try shutting of your incoming water source to the furnace so you can drain the boiler and remove the bleeder altogether. This way you can clean it out, as it is possible it has some sediment in it. A correctly operating system will just continue to refill any water lost from your bleeder also.