I use BD Ultrafine pins when using slin pins, and I never take the plunger out with them. They are the smoothest slin pins I've ever used. They pull like they have a larger gauge in comparison with other pins of equal gauge (and I use 31's for slins).
When I use regular pins, all of them are the interchangable screw off type. I always draw with one pin, switch, and pin with a fresh pin.Slin pins I can get away with drawing and pining with since the gauge is so small. Many people do this. I would never draw and pin with the same pin when using larger gauge pins though, and not because of sterility concerns, but because drawing with any pin will dull the head of the pin when pushing into the vial. Small gauge pins have less of a concern as they will not cause scar tissue, even if slightly blunted from using to draw. Using a slin pin is like pushing into butter. Smoooooth.