Mix n match isn't a problem but how about giving us a clue to the design and size etc. Also what are you intending to do with this machine.?

Heavy isn't always better.! . . . Done correctly then Mass can help but it's expensive to do correctly because to take full advantage it doesn't just stop at motors. What's the point of having a gantry built like tank when to take full advantage you need 25HP spindle.?
Any machine is only as good as the weakist link and I guarantee there will be more weak links in your design than the gantry, Also if too heavy or not done correctly then it soon can become the achilles heal of the machine.

The motor Calc spreadsheet is useless in the real world of DIY CNC, far too many variables come into play which the spreadsheet doesn't account for. The person who designed the sheet had never even built a CNC machine so couldn't posssibly know all the things which come into play.