There aren't any sensors for the ride height. The Nivomat shock controls everything internally to maintain the rear height. By changing only the rear springs to shorter ones, the car will be lower but the shock will constantly be trying to re-adjust the height and will be fighting internally to achieve it. For everything to work in harmony, the shocks and springs should be changed as a tuned set.
Now for the install question, check out this thread. It has some good info but the real meat is a link in post #3. It has two videos done by a good mechanic and it shows you everything you need to know to swap out the front and rear struts, shocks and springs. I thought it was great info. I haven't mustered up the courage to tackle the rear stuff yet but I swapped my struts basically the same way he showed.
2006 Inferno Red SRT8 with groups I, II, Sunroof, K&N drop-in filter, Auto-dim mirror, Jet-180 T-stat, Predator, 300 SRT struts/springs, Chally SRT wheels.
Born 6-21-06, Home 4-26-10. 1/4 mile - 12.989 @ 108.78 in street trim. Traded 10-19-2012.
2006 Inferno Red R/T with R&T Born 8-12-05, Home 8-25-05, Sold 8-9-10.