The general rule is to avoid mixing SnPb BGA and Pb-free paste or Pb-free BGAs and SnPb paste.

With the former you just have to be sure that the BGA and anything else on the PCBA will stand the reflow peak needed to melt the paste and merge with the molten ball.

With the latter, in principle its no different from soldering a RoHS exempt high melting point SnPb ball CBGA with lead-free paste - and that's unavoidable but they don't collapse either. However, if the rest of your PCBA will stand it, then getting the reflow hot enough to make PBGA Pb-free balls melt and merge with the SnPb paste is wise. I had a nice xray xsection image from our early experiments at Dolby that showed what happened when you don't achieve this.

Nigel