If the drive with your library isn't available when you start iTunes, it will default to the standard library location on the SSD. You just need to keep that in mind because it's easy to end up with two libraries accidentally. If you hold down the option key when you start iTunes, it will ask you to choose which library to use though.
I got a 512gb MBA in 2013 because I wanted to keep my whole library on the SSD. As my library grew, it soon became obvious that wasn't going to work. Finally I got a Mini and set it up with a big external drive as an iTunes server. This works really well with home sharing over wifi. I keep a very small library on the MBA most of the time. But if I'm travelling, I add more files to the SSD. My library is over 1TB and growing, so flash drives and SD cards are not really a solution for me anymore.