OP I don't know what your letter says as I don't have one starting this year - do you have to select another school as a back up? It would really depend where you are, as the catchment is now encompassing 4 former areas....
If you are up Hyndland side (and Hillhead's catchment goes quite far up there) then obviously Hyndland Primary or Broomhill if non-dom only. I have no personal experience of either, maybe others can advise. Though their numbers may be climbing too.
Though at this stage you might still get into Hillhead? This might be council's first letter out to check exact numbers?
As for the rest....I agree that there should have been some planning done with this merger, especially as thewaves says the old Hillhead's roll included many placing requests and therefore the council should have planned for their siblings - no-one wants to have kids in different schools.
However, there were still new starts who were non-sibling placing request and the council should have had the foresight to see how that would effect numbers and classes as it advanced up the school.
How all this is going to effect children already in the school and the new 2014 intake, I do not know.
Less space all round, problems with PE timetabling, loss of general purpose rooms, no library /IT suite. And, as mentioned, an old house sitting in the middle of the school that should have been compulsory purchased in the first place.