You are absolutely right about the fuse, the G540 will never, ever draw more than 7A. That being said properly sizing the power supply has a lot to do with the type and quality of power supply you are using. If you use a linear unregulated supply you need to 'oversize' it about 30% so that its output voltage remains at an acceptable level while under load. If you use a good quality switch type supply it should be able to put out its rated current and rated voltage with no problems.
You do not need/want big steppers on a Taig. I use/recommend steppers about 166 oz-in. I have been using this type of motor on Taig systems for quite a while now. Since the Taig has a 20 TPI lead screw the acceleration and velocity of the motor are more important that gobs of torque. Here is a short video of my 'little' motors running my Taig mill at 90 IPM.
[nomedia="http://www.youtube.com/watch?v=RzrBcXCNDhU"]YouTube- 90 IPM SS[/nomedia]