The best way to convert formats on a Sinar P is with different format kits that include a rear format frame, back, and bellows, and they come standard in 4x5", 5x7" and 8x10". For 8x10" note that the metering back, format frame, and bellows are larger than the non-metering versions of the same. The rear standard bearer has a peg that fits into a hole on the format frame, and the format frame clamps to the peg with a thumbscrew.
The front standard and lensboards are always the same.
There is also a larger 8x10" standard bearer, but many 4x5" P and P2 cameras already have it. I think the easiest way to distinguish the smaller one from the larger is that the larger has a knob on the left side when viewed from the rear that locks the swing movement on the rear standard.
There is also an 8x10"->4x5" reducing back that fits the non-metering back. I'm not sure if there is one for the 8x10" metering back. With a reducing back, you can't take advantage of the asymmetric movements on a P, but it's a handy way of shooting a 4x5" Polaroid in the midst of an 8x10" shoot, or using 4x5" with the longer 8x10" bellows.
As to whether you can use the lenses you have with other formats, it will depend on whether they have a large enough image circle to cover those formats.
There is no 4x5" positive/negative instant film anymore, except for what Polaroid Type 55 is still around to be had at a very high price. If you want to shoot instant film, there are a few varieties of Fuji instant pack film, but they do not produce a usable negative.