You say this as if it is a widely agreed-upon fact -- as if teachers generally recommend starting on short-scale and later moving to long-scale -- but that just isn't true. You could do it, of course, and there might be some teachers who would recommend it, but it isn't anything like a truism.
I learned on a short-scale bass for the first few months -- but I didn't know it. I had borrowed a bass from a friend that happened to be short-scale, but he didn't tell me that and I didn't know at the time that basses have different scale lengths. After those first few months I made my first trip to a music store with the intention of buying a bass, and it was one of the most frustrating afternoons I've ever experienced: I couldn't play a single bass in the store. I was like, "What is wrong with all these things?" I went home empty-handed that day, but some time later went out and bought a long-scale bass. What followed was months of frustration, as I felt like I was starting from scratch again. Eventually I got used to it, of course, but I think starting on the short-scale really set me back, and that I would have been much better off if I had started on a long-scale bass from the start.
I started out on a shorts cale Kalamzoo KB as a kid, and really didn’t even know there were scales. I went from there to a Fender Coronado Hollow body which is also SS still not knowing about scales. Years later I bought a Jazz, and then I realized there were scales because it was hell to play. My experience is much like Lobster11’s as far as frustration, and I have said many times on this topic that I believe it is easier to go from long scale to short than the other way around. In many bands I played in over the years, I also sang. And when singing and playing my Jazz I was always looking at my fingers on the fret board.
It took some getting use to but I can switch back and forth between LS and SS with no problem, but SS by far is still easier to play when singing. I own 11 basses, 5 are SS, and since I’m retired now from touring with bands, I pretty much play for myself or jam with friends. In most cases I will grab one of my shorties.
Since you’re a beginner you may not know which bass is the most comfortable, at least I wouldn’t have in my teens, so you might have to have someone pick for you. Some of the more seasoned bassist have given you some good advice, so I’ll throw mine in too. My 1st was a Kalamazoo which is a lot like a Epi EB0. It was easy to play and had few knobs for me to worry about tone shaping. It was solid in the mix but I was always tuning. I didn’t realize at the time it was a crap plywood cheapo with bad tuners.
Maybe a EB0 will work for a SS. If you go LG get a Squier P for the same reason. It sounds good in the mix and you can concentrate on learning to play. I personally do not recommend the SS Jag. Many on the site really like them, others have said it feels like a toy in their hands. I do own one, and it’s probably my least favorite bass, although I don’t hate it. The neck is very thin, and the string spacing is small. Try learning on that, and then moving to long scale, it will probably drive you nuts.
Whatever you buy, invest in a good setup!!!...........Good Luck
BTW, “Short scale is not good for rock”, is total crap. Noel Redding played a Mustang on occasions with Jimi Hendrix.
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