When I'm working on my speed (playing chromatics and stuff), should I be trying to keep my thumb towards the top of the back of the neck, or keep it in the middle (like a bassist) of the neck? I've got pretty small hands, and is it bad to play with your thumb on the middle if you're not doing wide-hand stretches?
keeping your thumb in the middle, as you rightly suggest, allows your hand to perform large stretches. the reason for this is that your hand is extremely relaxed, your wrist is probably in a position where you tendons can move freely and you have a better angle over the fretboard. i would say keep the thumb in the middle, just like a classical player because it allows for your hand to move freely and efficiently.

that said, i would also practice a bit with my thumb at the top for soloing purposes.

you just want to have control over what you're doing, the situation will not always be the same, so just practice how you feel comfortable and expand from there.
Put it wherever is comfortable for you.
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Thumb behind the neck improves your reach and speed, thumb wrapped or slightly behind the neck provides superior strength and control. Both have their uses and I suggest you master both and use them interchangeably.
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