If you add or take away peripherals later you have to be prepared for a fab spin in most cases since your peripheral I/O assignments will move around.
I, sometimes, 'add' a peripheral to the crossbar to have a 'hole'. A;so if you just do not need a peripheral you planned, just leave it on the crossbar. That, of course, may cost you a few port pins, but with an 8-porter, that is usually not a proble,

The SiLabs configuration wizard tool will help a lot in getting to an initial set of configuration code that you can follow in your firmware development.
it is an excellent tool and have amazingly few snags, but it has some, DO CHECK the result.

I think if they really wanted to be sophisticated about it that they would have made it as a full cross point switch with any peripheral targetable at any I/O pin.
I asked a top SILabs, naah that was Cygnal, engineer that question and the answer was "we tried that, it took 9/10s of a larger die than we wanted to use.

Erik