A few Contax tips.
Contax Claw- how you hold the pre-war Contax and Kievs. You probably already figured this out, or you are not focusing! Right Hand: index finger on shutter release; middle finger on focus wheel; Last two fingers underneath the right rangefinder window. Left hand more typical.
Always cock before changing shutter speeds. Not doing so may lead to lost shots.
If you are having difficulty composing (as you indicated), consider using a viewfinder. Personally, I prefer them. Many right eye dominant shooters have their right eye on the rangefinder window AND their left eye open scoping the scene. I have not tried as I am left eye dominant. Could be very useful for street shooting or shooting kids, people, animals, etc. I could probably do something similar with viewfinders (keep right eye open), but not as useful because you may not detect focus changes.
Lenses- Personally (and this is very individual), I prefer wide angle or normal lenses on rangefinders. The 35mm f2.8 Biogon is great (I have a post-war one). The 21mm f4.5 is also considered great- I have a Voigtlander SC Skopar 21mm f4 instead, and have really enjoyed using it. Of course with the German Contax rangefinders you will need a viewfinder to use wide angles (or any lens other than 50mm).
Great shot of your granparents, also. Really makes the camera special.