Yes, initially I had plans for another level of city growth requiring no AI planets within 2 hops and a second hab-center, but when I got to the end of the progression I realized that 5 cities with 6 nodes each was about right in that I only wanted it to support 1 dreadnought and 2-3 battleships.
So only one hab center is necessary for getting to the 6 node size.
Do note that hab centers now provide a goodly chunk of resources, though, so conceivably you may want more than absolutely necessary.
But the expectations is that players will want to absolutely maximize the number of spire capital ships, which means splitting all available space between reactors and shipyards
One thing to note is that a shipyard only requires that it's supporting reactor be somewhere in the galaxy, rather than on the same planet, so you can fill your "frontline" cities with 1 hab center and 5 reactors to get extra defensive modules, and fill your "backfield" cities with the corresponding shipyards. Those cities won't be as well defended by modules, of course.
FYI, just checked these in for 4.040 :
* Heavy Beam Cannons buffed:
** MkI beam count 1 => 3
** MkII beam count 3 => 6
** MkIII beam count 7 => 12
** All Marks armor piercing from 2000*mk => 8000 (flat, not multiplied by mark)
* Fallen Spire Stuff:
** Spire Shard Reactors, Habitation Centers, Shipyards, and the Galactic Capitol now have a primary armament; the Capitol's is similar to a Spire Dreadnought, the others' are between a Spire Destroyer and a Spire Cruiser. For all of them, the base range is roughly double that of the capital ships.
** Spire city laser-cannon and heavy-beam-cannon modules are now significantly more expensive and much more powerful.
** Spire city shield generator mkII health doubled to put it more in line with the mkI and mkIII (the ship-based versions used by Spire capital ships and Hybrids are intended to have a big jump from mkII to mkIII).
** Combined with the 4.039 changes, this probably actually makes things too easy on defense, but not trying to correct for that (possibly non-existent) problem yet.
The first bit about HBCs is for the base-game ones too, but is relevant to the Spire stuff since they use those quite a bit.