After giving this more thought, ooze probably is a good fit. The amoebic crawler from Dragon Magazine #330 solidified it for me.
Probably not. Let's try it that way and see if the need persists for separate statblocks.
Okay, let's start on a Small one and see how it goes.
I'll start by using a Grey Ooze's stats. The first issue is these are ranged attackers so ought to have higher Dex than most Ooze's have, otherwise they won't hit anything. I'll arbitrarily set its Str and Dex 10.
Giant Peridinium, Small
Small Ooze (Aquatic)
Hit Dice: 1d10+4 (9 hp)
Initiative: +0
Speed: Swim 30 ft. (6 squares)
Armor Class: 13 (+1 size, +2 natural), touch 11, flat-footed 13
Base Attack/Grapple: +0/-4
Attack: Trichocyst +0 ranged (1d4)
Full Attack: 2 trichocysts +0 ranged (1d4)
Space/Reach: 5 ft./5 ft.
Special Attacks: Blinding flash, trichocysts
Special Qualities: All-around facing, blindsight 60 ft., ooze traits
Saves: Great Fortitude, Iron Will, Weapon Finesse
Abilities: Str 10, Dex 10, Con 19, Int -, Wis 1, Cha 1
Skills: Swim +8
Feats: —
Climate/Terrain: Any aquatic
Organization: Solitary, colony (2-12)
Challenge Rating: 1?
Treasure: None
Alignment: Always neutral
Advancement: 2 HD (Small); 3-4 HD (Medium); 5-8 HD (Large)
Level Adjustment: —
All-Around Facing (Ex): Peridinium have no "front" or "sides". They can sense and attack in all directions simultaneously and can not be flanked.
Blinding Flash (Ex): All creatures within 10 ft. must succeed at a DC 14 Reflex save or be blinded for 1d6+1 rounds, those that save are dazzled for 1 round instead. Creatures beyond 10 ft. but within 30 ft. must succeed at a DC 14 Reflex save or be blinded for 1 round. Once a peridinium uses blinding flash, it can't flash again until 1d4 rounds later. The save DC is Constitution-based.
Trichocysts (Ex): Giant peridinium attack by firing spike-like projectiles called trichocysts from pores in their hide. A peridinium can fire multiple trichocysts as a full attack - 2 attacks for a Small peridinium, 4 attacks for a Medium peridinium and 6 attacks for a Large peridinium. Each trichocyst has a range of 30 ft. (50 ft. for a Medium peridinium, 90 ft. for Large) and does 1d4 piercing damage if it hits (1d6 for Medium, 1d8 for Large).
Peridinium have a limited supply of trichocysts; a Small peridinium typically has 40 such missiles, a Medium peridinium has 60 and a Large one has 100 trichocysts. It takes 2 hours for a peridinium to grow new trichocysts to replace any it has fired.