Yes, scrap solder would be better than pewter as a tin source for split shots, but...
1. The additional up to 3% antimony, copper, or silver content act as hardeners.
2. Acid flux core solder is corrosive to melting equipment and best avoided as a casting metal. Resin core will pool on the melt surface and emit lots of stinky smoke. Paper towels can be used to blot out most of the liquid resin(messy!), and the remainder can be flux burnt off.