Let's go with immediate and move actions.
Updated.
Skills: 4
Spot?
Feats: 1
Weapon Focus (spines)?
Well I don't know what their eyesight is like but they've got pretty big eyes, so Spot is OK by me.
WF (spines) doesn't feel right to me, since the real-world variety are not aggressive.
They're tough little fellows though, and have a nasty neurotoxin, so Endurance or Ability Focus (poison)?
Speaking of that poison, the real-world porcupine fish has poisonous internal organs, not spines. I don't mind giving the monster version poison spines, but feel we should keep the ingested poison for creatures that try to eat them. Maybe make the latter nastier.
How about:
Poison (Ex): A porcupine fish's spines and many of its internal organs are poisonous.
Spine poison - Injury by spines, Fortitude DC 12 [or 14 with Ability Focus], initial and secondary damage 1d6 Con.
Organ poison - Ingestion, Fortitude DC 12 [or 14 with Ability Focus], initial and secondary damage 2d6 Con.
The save DCs are Constitution-based.
Challenge Rating: 1?
Advancement: 2-3 HD (Small)?
A giant porcupine fish is 2 to 3 feet long and weighs x pounds.
The CR looks good, but I'd have them advance to at least Medium and possibly even Large. They are supposedly
giant porcupine fish, and the real-world variety can reach 3 feet long in the wild.
So, Advancement: 2 HD (Small); 3-4 HD (Medium); 5-6 HD (Large) ?
As for the weight, some googling has found that these fish range from 15-90 cm in length and weigh from 40-100 grams for the smaller (20 cm or so) species and 500-3000 grams for the larger (30-90 cm) ones.
I suggest we scale the giant variety to a 30 cm, half-kilo specimen. Scaling that up to 2 feet would give it a weight of 9.25-31.25 pounds.
Which would give us something like...
A giant porcupine fish is 2 to 3 feet long and weighs 10 to 30 pounds.