I have used male only and female only. Most trapping situations it doesn't make a big difference as to which is better. Unless you have extra holding pens I would mix them together. Better make sure your animals get along and that your pens are built to be chew and escape proof. They will work your pens over, you need to check them daily.
This time of year you have missed the best collection months.I have always collected mixed sex urine and done well with it.
The best diet is that of mixed wild game if available. Muskrats, Nutria, rabbits, groundhogs, venison, chicken are all good .
Dog food will alter the final odor,color and strength of the urine. I wouldn't add salt to their food personally. They will drink enough water naturally as needed, so be sure to keep it available. Keep your water cans outside of the pen so they don't spill excess water into your collection process and keep your pens covered from the elements if set up outdoors and not under roof.
Strain the urine very well and keep the feces out of your collection process and any blood. Good clean urine will last a long time, if not indefinitely, if stored properly. You can freeze it depending whether you have room for it and are allowed.
Summer urine, poor diets and warm collection temps can sour some urine over time..
Short term collection you may be fine at this time of year.