You work on landings your whole entire life. Every time you train, train landings. Period.
As to when you "can" move on to other skills, I'd say move on to them as soon as you feel comfortable with your body moving. When you feel "conditioned" for it. Your body will know when you're ready. I wouldn't work on just one skill at a time before moving on to the next thing if I were you. As to what to work on next, it's hard to say. The thing about training is, your training environment will dictate which skills you work on. If there is a wall there, work on wall runs and cats. If there are rails there, work on balance, vaults, etc. This, of course, after you've properly warmed up and drilled landings and rolls.

And "too tall/too small" is a bunch of crap. Like others have said, some things may be harder or easier for you because of your height, but the only thing that can truly make parkour training impossible is being dead.