In the link you posted they state the spindle speed as 8,000 to 24,000 RPM this is the minimum and Max speed that will have any useful torque you can do cutting with
The answer is No to both questions, the normal minimum speed for these spindles is 6,000RPM there are some that are better quality that can go down to 3,000 RPM, can you mill steel sure anything can mill steel even a Dremal hand tool, if you want to mill steel though, then you need to have the right spindle for the job 4 pole and lower speed that have a different Bearing arrangement than what these high-speed spindles have and are designed to mill steel