It is generally better to leave part of a leg retracted even if it means extending more legs. it is easy to see that the leg collars generally get more rigid with some leg left inside the upper section.
I know this is true for twist collars. I don't own any modern flip lock collars (such as Manfrotto uses) so can't speak from experience. But I suspect the general idea applies.
For a 500mm lens you should try to use legs with 32mm or larger upper leg tube diameters. They are hard to come by on a budget. On a quick google I did not see that spec listed, and that is not unusual. I would personally never spend much time even researching a tripod without knowing that spec because without it I have no idea of the "relative size" of the tripod. Payload ratings are just not reliable indicators of utility at given long focal lengths.