Likewise, how compatable are APAR and Aster?
Not very. Aster is a self-guided missile: It has a complete radar in itself. The Sampson array is mostly there to detect targets and beam early course updates to the Aster missile. But for terminal guidance, the Aster is on it's own.
APAR, OTOH, is a illumination radar as well as a (relatively short range) target detector. It functions on a much shorter wavelenght than Sampson. The upside of this is that APAR is very good at detecting small objects in places with lots of clutter, like a seaskimming missiel in the littorals. It has a high resolution. But you ALSO have to use it for illuminating your targets when your missiels are getting close, because all but the latest Standard missiles only have a reciever set build in.
Because of it's short range you have to pair APAR with a large volume search radar. Sampson, OTOH, is apparently so capable in the volume search mode that the British are wondering if they could have done without SM-1850 altogether.
If you're using APAR, then slinging missiles like Aster is a waste: They are much more expensive than reciever-only missiles.