Would a 650 Watt Molequartz Mini-Softlite work as a Key for BetaSP or Digi? I am not sure if it has enough punch and on the MR website it is described as a good fill. Any real world experience out there?
Remember that beauty is in the eye of the beholder! For that matter any light can be a key light, but it's success as such depends on the quality of light you wish to portray as well as the other ambient light conditions.
The small softlight you refer to has a rather small oraface, maybe 8" square, so even though it is a softlight it will still cause a shadow when used close up, which might be the case because it doesn't put out much intensity. I often use larger softlights like the Mole 2K zip or full sized 2K or Baby 4K (same size) as a key light on a stage. These units create a similar quality as a junior or senior aimed through a diffussion panel like silk or Lee 216, but not quite the intensity and are faster to move around if there are many setups.
Again, it depends on the ambient conditions too. If you're in a typical day interior where there is spill light from windows, the soft light is the wrong color temperature and if you correct it with CTB gel you'll reduce its output to almost nothing.
To me it sounds like you already have one of these units, or are thinking of buying one. I'd advise getting a more flexible light that can serve several purposes like a Mickey (1K) or even a Teenie Mole (650W). These light can be used direct for a hard key, diffussed or color corrected with gel on the barndoors or with the proper speedring you can mount a Chimera or other light bank. Another good unit for portraiture is the Kino Flo 4Bank or Diva 400. These have the advantage of being able to switch out the tubes either daylight or tungsten, and are cool and efficient.
You'll find, even by perusing the Mole website that are many, many types of instruments, all designed for special purposes and applications in theatrical lighting; there's a good reason for the many choices! Obviously the challange in starting out is to select a kit of units that can serve double duty. Good luck doing so.