I really think you can. I almost beat the game without Atlantica until I remembered I need something from there to get some items from the Moogle mix shop thing.
I don't know if this is what you're wondering, but I could have sworn you got the ability "scan" earlier in the game than ursula for KH1. I think for every battle it is small and with Atlantica it blends in. In KH2, it is a musical and of course you cannot get the "scan" ability. In KH1, I got "scan" before defeating Sabor in Deep Jungle.
Atlantica
Arsissa atlantica was created in 2009.
Nephropsis atlantica was created in 1882.
Atlantica Party was created in 2005.
Emblemaria atlantica was created in 1898.
Starksia atlantica was created in 1934.
Cupressus atlantica was created in 1950.
Magnapinna atlantica was created in 2006.
Mastigoteuthis atlantica was created in 1933.
Opus Atlantica was created in 2001.
Clathrina atlantica was created in 1908.