You don't talk about more branches the better in 4 in 1 trees- you just don't have a choice. I hate these trees- they are a virtual impossibility to train well. Each variety has it's own level of vigor, but in Asian pears and peaches this might not be as big a problem as with apples. Apples are all I have experience with in this form. It looks like branches were already cut back for shipping. What you need to do during the growing season is keep the 4 different varieties growing at about the same pace so one doesn't dominate the tree. I can see that will be an issue for the peach for sure, with the top branch in position to dominate. Pinch growth that eminates from where branches were cut to encourage dominance of a single leader while developing less vigorous (smaller) secondary branching off it. Each variety should be treated like an individual central leader tree in terms of form. The central leader being like a trunk with smaller branches (no more than half diameter) coming off it. Also pinch back as needed to keep the different varieties in balance. Pinching means that you're removing as little growth as possible to stear energy where you want it, so it won't really retard overall growth. If you let the trees go without guidance for even one season you may loose control with certain varieties dominating the tree and starving out other varieties. Anyway, that's my opinion. My experience is primarily with 4in1's that have been badly trained and I have to try to straighten things out later. I would never produce a tree like this, much better to alternate varieties by tiers off a central leader, placing most vigorous varieties at base. In other words, new varieties are grafted as extensions of the trunk as the trees grow up. This wouldn't work well for peaches, though.

I would let them branch out at ~6ft. Dragonfruit do not need to be a certain height to set fruit, its more of a mass thing. If you want it to fruit, don't give it too much nitrogen several months before you expect it to flower, instead, give it mainly phosphorus. I recommend letting your dragonfruit grow up a post. You can look up pics they have in Vietnam, it looks like a normal tree. If you let it grow on your fence, it will fruit in your neighbors yard.

What you have is a whip - a very young tree with not yet any lateral branches developed. The suggestion you received is pretty standard for newly planted whips and is worth following as it encourages the development of a strong branch system. And that's were the fruit will grow. What you want to do is make a heading cut where you want the branching to begin and just above one of the buds. You do not need to take it down as far as they said - that is normally the practice for semi-dwarf or larger growing trees. And you don't want this tree to start to set any fruit until it develops a proper habit with some branches and a basic structure. So remove any flowers for at the least the first few years. And you should continue pruning in subsequent years but not in this same manner. This is pretty much a one time shot. You might want to research apple pruning or maybe a local nursery or your extension service has classes you can attend. Apples need routine, preferably annual, pruning to stimulate and encourage the development of fruiting spurs or those little nubby protuberances that the fruit grows on. 6-8' is really short for a dwarf apple. I'm thinking this is a mini dwarf, which only get around 6' tall - dwarfs typically get about 8-10'. Do you have another apple for cross pollination?

In my opinion, for the 2nd year tree, I would either remove flower buds now or let them flowers (if you want to see how beautiful it looks) and remove all flowering clusters after that. Such a young tree should waste energy to produce fruit in its 2nd year. Let the tree uses its energy to grow and establish. You'll get better a return of your investment in the long run. Also, young trees often abort their fruitlets when they are not ready. I'd rather help the tree now before it goes that far.