re: Poker & Pot Limit Omaha versus No Limit Hold'em
There are some principles that apply in NLHE and PLO. For the most part the basic principles of pot control and equity etc. can cross back and forth.
What I would say is you need a larger bankroll at the stake level you are playing at to play PLO. The game has more variance in winnings and losses, even for a winning player.
PLO requires more finesse in how you approach the hand, you have to play a tight range of hands, but it is a very different range than what you would play in NLHE. One of the reasons it is difficult to move from NLHE to PLO is that it is a harder game to master and some of the principles from NLHE transfer well and others (hand selection and hand strength) absolutely do not.
PLO is a tough game, but it can be profitable. Because it is a hard game, you can really do well in it if you understand how to play it. Most players (especially at the low levels) have absolutely 0 comprehension. That means a little bit of an edge can make a big difference.
Skill level is not an easy thing to gauge- and there is not "level" where going from NLHE to PLO is easier or harder. They are different games with enough similarities that it can be hard for some people to transition.
PLO is also a game that has been less studied, which means it is harder to get an edge in.