In summary (and ignoring road bikes which do something different again) you can have:
Internal diameter of (I think) 34mm which is for external headsets with 1 and 1/18 steerer forks.
xx44 with 44mm internal diameter top and bottom which will allow 1 and 1/8th steerers fir internal headsets, tapered steerers with upper internal headset and lower external headset or 1.5 inch steerers with upper and lower external headsets.
50mm internal diameter headtubes will allow all 3 steerer types to be used with the appropriate cups.
Then there are 2 types of tapered headset (E2 and Scott/Lappiere) which allow tapered to be used with internal headsets, I and 1/8th steerers with an insert into the bottom cup.
Glad we have a headset standard to make sure it’s not really difficult.
EDIT: In respect of the Piglet I don’t think it will make loads of difference and I am still pondering what to buy. I want it to be as future proof as possible but they will probably change everything again as soon as I buy something.