Whole Psyllium husk (Frontier brand): fiber source, must add at least 1 Tbsp water -- 1/16 tsp
L-Glutamine (Source Naturals brand powder): an amino acid shown to help slow down catabolism (break down of musle) and repair the abdominal wall and mucous lining -- 1/16 tsp
Saccharomyces Boulardii: increases gut immunoglobulin, Secretory IgA (SigA) in duodenal fluids & anti-inflammatory cytokine, Interleukin 10 (IL-10). Thorne's is a good option, but for cats with constipation, Jarrow is better because it also has MOS. Reseach summary on S. Boulardii here. -- in children's studies, 250 - 750mg doses have been used, and in adult studies 1,000 - 3,000mg. For a cat with acute symptoms, one would use more but for a chronic situation with pudding poop (as opposed to really loose stools:), 50mg can be enough. Cut back on dose as symptoms e.g. firmness of stool, improves.
Digestive enzymes (Prozyme, Petguard, or Dr. Goodpet): cellulase, lipase, protease, and lipas, not to be confused with proteolytic or pancreatic enzymes -- follow brand guidelines.
Fructooligosaccharides or FOS, a prebiotic: Note - some cats can get gas and bloating from this, so be watchful. I would avoid this for cats prone to loose stools. Also if feeding canned food, many of them add inulin/chicory as FOS, so do not add extra if this is the case. Piggy was (still is, and will always be) on a raw diet, so she didn't get FOS from her meat diet. -- a little goes a long way on this one, amount will vary by brand.
Probiotics (Jarrow Pet-Dophilus powder or non-dairy UAS Labs DDS capsules which also contains FOS) -- look for as many strains of beneficial bacteria as possible and a minimum of 1 billion CFU per kitty serving.