If you have a 29 gallon, you can have one upside-down catfish. Make sure you get a 29 gallon long so that your catfish has lots of room to explore.
The smallest tank for a sorority is 20 gallons but if you want to keep an upside down catfish, a long tank is the best, preferably a tank of 29 gallons or larger. Upside-down catfish should be fine in a sorority because they are nocturnal and you will almost never see them out and about during the day. Upside down catfish need a lot of hiding places. I have natural lace rock, which has a lot of holes and caves, for my upside-down catfish. They are very active at night but during the day they need a secure place to hide.
Definitely do not risk any sororities in a 5.5 or a 10 gallon tank. Sororities need a minimum of 4 to 5 girls. In a 29 gallon, you could safely add 6 or maybe even 7 depending on what other catfish you add. The more females, the less aggression there will be.
Do you have a liquid kit for parameters other than pH? Testing only the pH is not enough and it does not tell you whether your tank is cycled or not. PH is one of the least important parameters to check and it is only truly necessary to test pH when you are breeding fish. What are your readings for ammonia, nitrite and nitrate in each tank? Do a fishless cycle using surfactant-free ammonia and check to make sure you have 0 ammonia, 0 nitrIte and more than 1 ppm nitrAte before adding any fish. 0 nitrAte means your tank is not cycled and it is not safe to add fish.
Buy all your females at once. It's normally not a good idea to add more than 3 fish to the tank at once, but you must add all the females at once for sororities. When you have the girls at home, sit their cups in a cluster on a table for 15 minutes so that the girls can look at each other. Then, float their cups in the tank water for 30 minutes while you slowly drip water from the tank into each cup. Then add all the girls at once. Allow them to explore the new territory. You will need a lot of hiding spaces for the girls. Small flower pots without holes in the bottom are cheap and bettas love them. If you can only find pots with holes, use aquarium silicon to seal the holes shut. Allow the silicon to cure outside of the water for longer than the minimum recommended time on the bottle. You will also need a lot of plants, whether they be live or fake. Live plants benefit your tank a lot more than fake ones, and they look a lot nicer. Some easy-to-care-for live plants include java fern, java moss, hornwort, anacharis and hygrophila. There will be a lot of aggression for a while as it is unnatural for females of different spawns to live together. The girls will determine a pecking order after a while and the dominant female will lead the sorority.