i want to know if this http://www2.towerhobbies.com/cgi-bin...?&I=LXHHC2&P=0 servo will be good on cap 580flatout

Slightly on the heavy side, IMHO. If weight counts 6-gram servoes is better. But it have more than enough torque for the job, no problem

hs-55 is good?

Hi Oren! HS-55's will work fine in a Cap 580 Flatout. I use them in all my Flatouts, and the only time I've noticed any type of problem is when using them in the biplanes. They still work, but don't seem to center well. Good luck!
Trevor

Try United hobbies.com 9 gram servos. They are better than Hs-55 and only cost half as much.