Differences in color temperature would depend on which strobe, speedlight, and softboxes are being used. Particularly in the cases of the softboxes, different brands -- especially the less expensive ones -- can have color differences due to the fabric used for the diffusion panels.
For the strobes, some models will have significant color changes at different power levels, while others -- along with speedlights -- are more constant in color due to using full voltage that is time-quenched rather than variable voltage to control exposure levels.
Because of the variables mentioned by Brian, you may want to keep gels or other color-correcting materials on hand. The fill function is particularly noticeable when its color is out of whack versus the key. For rim, hair or b/g purposes, color imbalance is much less an issue - unless the application is particularly technical.
Gelling two different-colored lights with the same gel produces two gelled lights with different colors.
I've used speedlights with monolights on a number of occasions, including at weddings and in doing portraits. In portraits, I'm usually using the speedlight as a hair light or kicker, or possibly as a background light. I've never had a problem with noticeable color differences. I haven't tried using a monolight as key and the speedlight as fill (or vice versa), where a color difference might be more noticeable.
For me, I've done something "similar", and I've also mixed/matched brands of flashes and strobes.
The differences weren't enough for me to care, and I could get the color "close" enough. I'm not as picky as someone like Lee Varris (I think is how you spell it) who wrote Skin
If I'm mixing/matching, I just make sure to take a gray card reading from the final set up. Then again, I'm not entirely convinced my lastolite ez-balance is completely accurate either lol.
