About Shadow, just keep playing. You'll get your answer.

About everything else before the Floating Continent, you might go get Mog's Water Rondo. Put Mog in your party and set the airship down on the Veldt, then go through the Serpent Trench like you did before. Once again, you'll end up in Nikeah, except this time you can head east (instead of just going to Narshe). Follow the narrow path along the shore, and pretty soon you'll end up in familiar territory (the crazy old guy's house, Doma Castle, etc.). Simply follow the same route you did in Sabin's scenario, and you'll get back to the airship in no time.

And if you haven't already, go get the goods from the Imperial Base near the Esper cave. Lots of good stuff abound in there.

Triangle Island is a tough place to level up, since the only legitimate creature there is Intangir, hard to kill. You have to un-Vanish the guy, hit him with everything you've got, then heal from his attack before repeating the process again and again. You get 10 Magic Points for winning.

About Espers, go get Golem and ZoneSeek from the Auction House in Jidoor if you don't have them yet. It'll take a little bit of patience for them to come around, but they're nice to have. Also, you can buy Sraphim from a guy hidden in the trees at Tzen for 3000 GP, or else you can wait for the WoR to buy it for 10. The choice is yours.

Level 26 isn't a bad level at all to take on the Floating Continent. I'd say go for it when you're ready.