I have had P90s in different types of guitar, and imho they sounded best in LP types. Donīt have an SG though. Thing is that compared to humbuckers the P90 type provides a deeper low with excellent definetion end an more open, higher high end (which can scream without gettin harsh as it stays natural), as they are single coils. Additionally the midrange opens up and breathes, getting a very special roar on top of it. What I found was that this character works well with LPs which tend to have a dense midrange and often less prominent high end compared to other guitars. Plus the scale length does not give the deepest low end. Since SGs are known to be midrange monsters I would recon the same applies for them.
P90s can also go into other types, like Teles, or Superstrats, but I found them to be a lot more picky there.
