the 621 will work just fine on a 10ft dish, providing it is a good ku compatable dish, an just like Lak said fine tuniing is the secret to get them to play. I got a 10ft Channel Master here with the 621-2 on it an I don't miss any transponders on 123, an get fine signal on the 125 sat even on the S-2 mux. also do you're self a favor an put a disqec switch in, an ditch that short jumper, an for c-band make sure that the coax is in the REC port on the side of the lnb, an ku band is at the back of the 621.
now the 621 have no F/D markings on them so you will need to measure a feed with F/D scale with a ruler, an set the 621 to it, then never change it. anothe trick that I have done is after you got the F/D set in the scalar, put a hose clamp on lnb throat with a couple pieces of small metal right next to the scalar ring as a stop, now you can set you polarity with out changing the F/D. then just remove it when you're done. the F/D is VERY important setting.
now what brand of 10ft dish are you trying to set up cause I have F/D & F/L settins for alot of dishes?