No problem mixing them, as the lead will alloy nicely with the extra tin and will still form an alloy that is somewhere on the curve. It will mely at a different temperature, and will be smooth unlike lead free.
NO!
Never ever mix leadfree with leaded solder. solder is eutectic . meaning it has 1 and only one point where it goes from liquid to solid. to get this eutecticity the mix ratio between tin and lead is important. only 63/37 is eutectic . any other mixture is NOT. even the 60/40 is not eutectic . the band is about 4 degrees C. in this band the solder is brittle. Any mixtures that go even further away from 63/37 are even worse and this may lead to cracking , whisker growing and all other kinds of instability over time ( tin pest , blooming etc ).
So : either REMOVE all the old in/lead using a vacuum sucker and solderwick , or solder with 63/37.
the same goes for those special kits being sold to easily remove tqfp's. that 'special' metal is actually bismuth . this melts at ver ylow temperature ( 140 degree i believe ) and stays liquid very long before it solidifies. that's why you can easily desolder stuff. before placing a new part you must remove all trace of the bismuth using solderiwck , re-wetting using regular solder , taking that waya using wick and then placing the new part. any bismuth in your solder allow will mak it so soft that vibrations o the board may shear things off the pads ...
2T4p. SAC305 is the biggest pile of tripe out there ... it is intended for automated, controlled , mass production. NOT for rework. Never ! Jamais. there are tons o better alloys out there with MCM ( Mitsubishi Chemicals ) having the SN99c and SN100c alloys. Those work perfectly, have been used for at least 10 years in mass production with excellent results.