What brand of X-stand are you using? It looks like there are bolts to hold the height adjuster solidly in place?
I use my keyboard's music stand as a tablet stand, too.
Most brands of foot controllers should be compatible. The type of pedal is usually more important than the brand.
Sustain pedals are often simple on/off controllers (like the Yamaha FC4A and FC5), but some can also be used for "half-dampening" (like the Yamaha FC3A). However, I don't think you can use a half-dampening pedal with a given model of keyboard unless it supports that type of sustain pedal-- which apparently the PSR-S970 doesn't do, since the FC3A isn't listed among the compatible foot controllers, whereas the FC4A and FC5 are. So if you decide to buy a non-Yamaha sustain pedal, make sure it's just a simple on/off pedal rather than a half-dampening pedal.
As for the volume/expression pedal, I think just about anything should work fine.