Gates do a nice wee sizing calulator/programme, helps getting suitable belts/pulleys etc if you know a few dimensions of the machine, it is called Design Flex, you will find it here
http://www.gates.com/catalogs-and-resources/resources/repository/engineering-business-applications/design-flexRegarding buttons,
Spindle should go via Mach if Mach is controlling the spindle on/off. You can do it a few ways with the IP-A, you can use a Brain to look at the switches input and then have it do the spindle toggle, you can use the macropump to do the same or you can set up an OEM trigger to the port and pin number of the switch and under system Hotkeys enter the OEM code for spindle toggle. Personally I use the macropump but the other ways will work fine, just don't use the inbuilt modbus via brains or macropump and at the same time use Port and Pin via Machs Inputs page as you may get a conflict.
Back gear can be handled under spindle pulleys from Config menu, just have to choose reversed option for the back gear and set the ratio. Should work fine although I have never used it with the CSMIO.
You could do that for the ref button, you would have to do it either via the macropump or a brain, when it sees the button press it will look at the inputs from the rotary and do whichever option the rotary is set to. I wouldnt bother myself as if I ever wanted to just home one axis I would just use the screen but everyone is different.
When I had a VFD on the CNC BP I used it to vary the speed and just found a happy medium on the vari pulley and left it there, it was good enough for most things and if doing heavier stuff I would be in back gear anyway.
Hood