When the .357 SIG came out and the reloading articles followed, the conclusion was that you could resize .40 brass and yes, it would give you a short case most notably in the neck since you need additional length to form the bottleneck. The consesnsus oppinion was, more trouble than it's worth since the .357 SIG is short on caseneck to begin with. They didn't say, never try this at home.
I am particular about the brass I use and never use mixed or rangebrass, but if you have found a way to make an inexpensive load doing so and understand that you should never use it for anything other than plinking, more power to you!
RangerMonroe, what powder and primers are you using? Maybe a general purpose type service powder could help. HS-6 or something similar comes to mind rather than say VV 3N37 which is very good for holding flash to a minimum for defensive loads. I am not a big fan of using powders faster than say AA#5 or Unique for the medium cases of the .40 and .357 SIG. If it works for you, again more power to you!