No, amps channels are all individual, you cannot affect other channels by how many you hook up or whatever. If an amp is rated to deliver 4 channels of 75 watts RMS @ 4 ohms, then each of those channels can do that, and you can use 1 or all of the channels or anything in between.
For your first question, yes. But say you run into a dual voice coil sub that you want to buy, you will need to know how different ways of wiring it up will affect the impedance your amp will see. Here's a good example...
http://a248.e.akamai.net/pix.crutchfield.com/ca/learningcenter/car/subwoofer_wiring/1-DVC-2-ohm-mono-low-imp.jpg
This is called a dual voice coil (DVC) 2 ohm sub. It is called 2 ohm because both of it's voice coils are 2 ohms, not because the load your amp will see is 2 ohms. When you have 2 voice coils, depending on how you wire the sub the final impedance will be different. For the example above, the impedance the amp sees drops to 1 ohm. But the link below shows that with the same dvc 2 ohm sub, you can wire it up to be powered at 4 ohms
http://a248.e.akamai.net/pix.crutchfield.com/ca/learningcenter/car/subwoofer_wiring/1DVC_2-ohm_mono.jpg
But in the meantime, try to determine how much power you want/need, then match subs and amps. Example, say you want a 300 watt RMS sub, and you find one you like that is a single 2 ohm voice coil, or maybe a DVC 4 ohm (which can be wired to be 2 ohms, refer to the links again), then you need to find an amp that can put out that much power at that impedance. Amps will list their power outputs at a couple different impedances, and the lower the impedance of the voice coil(s) wired to the sub, the higher the power it can deliver to them.