Yes, you can certainly leave the tweeter in place and just cut the connections. You just don't want to leave a hole in the speaker baffle.

Those Warehouse speakers (formerly WGS) are really nice. I had one that came as standard in my Two Rock combo and that sounded lovely. I upgraded it to a Celestion Creamback (the Two Rock suggested upgrade) but it didn't really make any difference to my ears.

You obviously loose a bit of bass end with a 10" speaker compared to a 12". It's an open back cab, so you aren't going to be able to extend the bass response like you can in a ported bass cab. Unfortunately WGS don't show any frequency response graphs or give any frequency limits, which makes it hard to compare the difference between the 10" and its 12" equivalent. The 12" is about 1.5dB more sensitive/louder which is going to be almost inaudible. You've really got a choice of 8 ohm or 16 ohm versions of the speaker (it's a valve amp so the impedance won't make any difference to the volume). I'd avoid 4 ohms as it stops you using it with an extension cabinet.

...Which you may never, ever use, but if you do, you really want to have the extension cab the same rating as the internal speaker. 1x 12" extension cabs tend to come in 8 ohm versions if designed to pair with solid state amps (to give 4 ohms combined impedance - the lowest most SS amps go to but at which they produce more power), but 16 ohms if designed to pair with valve amps. 4x12" cabs are almost always 16 ohms (some switchable to 4 ohms for use with SS amp heads), so I'd lean towards 16 ohms as the most overly compatible value - and if you want a 15W amp to sound significantly louder than with its internal speaker, then a 4x12" is the only way to go. You don't want to mismatch impedances as the speaker with the higher impedance will take a lot less power than the other one, so won't be as loud, whilst you are stopping some of the amp power going to the lower impedance speaker, making that quieter. So then you are normally best either sticking with the internal speaker or (if the external speaker has a much higher efficiency) disconnecting the internal speaker and just using the external one.

But never use a valve amp without a speaker (or correct impedance attenuator/load box) fitted, and always make sure that the correct impedance output tappings on the amp are used.