Would 5.5 x 2.1 mm male plug work inside a 5.5 x 2.5 mm female jack for DC power?
I have tried fitting a 2.1 mm barrel plug in a 2.5 mm jack (by accident or consciously) many times and it has never fit. Both sides are rigid and there is not enough clearance.

Size adapters are available; I recommend having some around for use when you have a device that doesn't come with a power adapter and an adapter with the right voltage. They do have the disadvantage of making the plug longer, hence more awkward and vulnerable to damage.