Does tripod's load capacity include the use of stone bag/weight bag too? should I consider the weight that I may hang from the tripod as part of tripod's load capacity?
Legs, yes; head, no.
Most tripods (when new, at least) will actually support more than their rated limit comfortably in "normal" position, where the legs are splayed out somewhere around 20 degrees, but not in the wide stance (if it has one). With compacts and travel tripods, where the tubes start getting pretty thin, you'll want to avoid overloading them when the legs are fully extended, so leave the smallest tubes collapsed if you don't need them. As the tripod gets a little older, though, you can expect some slippage/creep in the leg locks or the centre column lock if you hang too much weight. (And of course that's going to happen when the mirror or shutter gives things a friendly little nudge, so everything will seem copacetic until you try to take a picture. Life is like that.)
But be reasonable. You don't need to hang more weight than it takes to dampen the system. You can add extra "stay put" stability by weighting with sandbags at the feet if you need it.