Read the SIM chapter to see how these chips work. Basically you can either connect a 32,768Hz crystal and use the internal PLL to generate the clock (at whatever frequency you want) or you can generate the 16MHz, 20MHz, 25MHz (or anything you want) externally and feed that in.
External oscillators have to have very well controlled duty cycles, so the 32,768Hz option may be easier unless your system requires the frequency stability of a high speed clock.
There is no "frequency locking". Different grades of chip are guaranteed to run up to a specific speed. You can try overclocking, but if it fails or is unreliable, that's on you. You may be able to overclock over a limited temperature range. Or maybe not. Basically if you're doing this for a hobby, then you can overclock as much as you like. If you're making something for production or sale, then you can't guarantee it will work reliably if you overclock it.
The tables in the SIM chapter give the programming for different frequencies, and shade the sections that generate clock speeds above the limits in a different colour.
Tom
