That's very different.
What I suggested deals with iTunes, which is what you asked - and yes, that deals with one song. It wasn't clear that you meant this for every single song in a playlist and in addition, on a selection that is no longer being managed by iTunes. Those edits (in iTunes) would copy over to an Apple portable device (iPod, iPhone) if the device was synchronised with iTunes, which again, is still effectively under iTunes' management. If the Apple device was being managed manually, those edits would not copy from iTunes.
However, it will not apply to a song (or songs in a playlist) once they are copied outside of iTunes, including to a memory stick. The key thing to understand is that iTunes is managing that stop time.
I understand what you want to do, but I think it's impractical when talking about a significant number of songs, let alone 600.
To achieve what you want would mean editing each actual song file, using an audio editing programme such as Audacity for example. As an aside, you would then be faced with the question of what to do with a song that has a long fade out; do you stop the song near the start of a fade out, towards the end of a fade out or part-way through the fade out, any of which (take it from me) can be jarring?
Returning to iTunes, there is an option to crossfade songs, one into the next and that can be adjusted to between 1 and 12 seconds. But it's a blanket, every crossfade is the same time, which would mean that sudden ending songs and long fade out songs would have the same crossfade time. It's the reason that I don't use the crossfade function in iTunes. I don't think there is a crossfade on Apple's portable devices.
I am aware of a "gapless" function (which is similar to, but not the same as, crossfade) in a music playing programme for Android devices, but on the portable device, it's still managed by the Android app. Gapless playback is effectively for songs on an album that the artist has segued (i.e. specifically led from one song to the next, without a gap).
I think you're asking for the impossible (unless someone knows of a workable solution of which I'm not aware). Having said that, there is one method, which I'll come to in a moment, but I doubt that it will help you.
It's a similar story with the volume of songs. There are solutions, but they're not very effective and once again, to manage a large number of songs on a memory stick, it would involve manually adjusting each song individually. But then what happens when you adjust several songs and then come across a song that is so different that it doesn't fit in with the songs you've already processed. You would have to compromise or start again from scratch. Why is it this way? Well, for two reasons, technical limitations and artistic choice.
So what is the "one method" that I referred to above? The answer is (or may be) DJ software.
Radio stations and online streaming services effectively manage both of the tasks you are hoping to gain.
Before computers, the radio station DJ (presenter) would manually adjust the volume of the playback of the song he or she was about to play and then the station's equipment would add additional control, usually by compressing the audio. Likewise, the DJ would manually control the playback of a song so that either he or she spoke over the end of a song or manually managed a segue from one song into the next. That was their skill.
Now, with modern computing, the software manages all of this, although not always well. I suspect that someone still has to programme every song to tell the software how to deal with each specific one.
It all comes with a compromise through. Sound quality can be reduced and certainly the artistic effect of different volumes is taken away.
So DJ software might achieve what you want, but that's on a computer. Perhaps there's an app that could do this on a portable device such as an iPhone or Android device, I don't know. But even if there is, I suspect it will be relatively expensive and once again, it won't manage files on a memory stick (USB) as it would still need the app to manage it.
One final solution is streaming services. They all (as far as I know) provide consistent sound levels (volume). Not sure about the gaps though.