1. For a beginner coil you do not need to twist the wires, the coil will work well also without the twisting. But if you want, you can twist it. The tighter the better. I would recommend about 1 twist per inch, that should do good.
2. I usually take my wires outdoors and wind them out till I have the wire of desired length. Then I add some more strands of wire to get the count I need for bifilar or trifilar coil . Then I simply attach one end of the strands to something and fasten the other end to a electric screwdriver and start twisting
3. You can wind all the strands with the same wire gauge, that will work fine and will be easier to twist than if using two seperate wire gauges.
4. Extra winds will not affect the circuit and you can use them later as slave coils with a separate transistor for each coil, or you may use two or three strands in parallel for powering to get less resistance and thus a little more power.