I planted these cuttings two months ago and I have a lot of questions.
- Is it time for repotting?
- I have 26 liters pots. Is it enough?
- Can I plant more than one in the same pot?
- What kind of soil should I use?
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These plants are in the genus Hylocereus which are jungle-dwelling cactus. They usually grow within or on top of trees and bushes and grow to be very long (30+ meters). In habitat they don't necessarily need to be rooted in the ground/soil. They are epiphytic, meaning they grow above the ground, supported non-parasitically by another plant or object, and deriving its nutrients and water from rain, the air, dust, etc.
So, How do people grow them as hobby plants? They are generally grown in smallish pots and allowed to climb and drape over whatever is handy. As a hobby plant they don't have access to trees, bushes, decaying leaves or any of the things they normally encounter in habitat. You have to supply for their needs and the easiest way is through the roots, in a pot.
All that said, I don't think you need to repot these plants at this time. I would wait until they are at least two meters tall (they don't get much/any thicker than they are now, they just get long). You'll need to secure them to something as they get tall so they don't tip over.
Another note about this genus is that because they are jungle cactus they are very frost tender. They will die if exposed to cold temperatures.
These plants are also used for grafting as the stock (the bottom part) because they root easily and take grafts fairly well.
To your questions: No, I don't think it's time for repotting. A 26 liter pot would be too big. 4-6 liters at most should do fine. You can plant more than one in a pot, but you'd need a bigger pot. The problem I see is that multiple plants in one pot is going to grow a LOT of cactus mass as they all get longer and longer.
As far as soil is concerned, you can probably use either cactus soil (if it's available where you are) or regular potting soil. Try to avoid potting soil with too much peat. You can give these plants more water and if you fertilize regularly (at half strength) you will get flowers and eventually fruit. They will tolerate full sun, but do better with partial sun and indirect light (like they'd get if they were inside the canopy of a tree).
I've seen these grown commercially and the set up I saw was a long trellis like those used for grapes or raspberries with dozens of stems running the length of the trellis.
I hope this gives you something to start with. The good news is that there is nothing you need to do immediately.