Most Gibson and Fender short scale basses are 30.5". There are a couple that are 32" but it is pretty rare. A few that are shorter like the Pocket Bass (I forget maybe an Airline or something like that), but they are super uncommon and not great because the string tension is too floppy and they just sound flat. The 30.5" length seems to more or less be standard for short scale. I have seen 32" referred to as medium scale sometimes.
As far as playability and muscle memory there isn't as huge a difference as you would think. IMHO string spacing makes a bigger difference in this regard than scale length. I think guitarist and bassist develop a sense of relative tone that sort of overpowers muscle memory. It might take a minute or two but adjusting from one to the other is as effortless as switching from a Les Paul to a Tele once you have been playing a little while.
The main difference to me is that the longer scale has a snappier sound (almost more like a low note on a piano) and the shorter is a little hollow and deeper (more like an upright bass). It's subtle but you start to hear it when you play each for a while. If you want to do, for example, that whole Flea thing then you want a 34" length. Likewise if you want a deep hollow upright bass sound from an electric then use a short scale bass with a mute and you are as close as you are going to get with out playing an actual upright.
And yes OP the affinity has a thinner body than the higher end ones. It actually makes it a pretty natural move for a guitarist, because the body is pretty much as thick as something like a Strat or Tele. Neck dive wasn't a problem for mine, it had smaller closed tuners that probably didn't weigh as much as standard Pbass tuners. Though neck dive isn't something that has ever bothered me so maybe I just didn't notice.
The Gretsch is pretty cool though. Another suggestion if you want to consider a short scale is the Squier Bronco bass. You used to be able to get them for $100 used, not sure what they go for these days though. The entry level Epiphone SG bass is alright too, but typically they need a little work before they are good players.