In my previous post, I outlined a science fantasy setting called Dreaming Stars - scattered colony worlds, each with their own unique human subspecies and cultures, which are now linked again by psionics capable of transporting people to the Plane of Dreams and back again. As noted, the setting is heavily inspired by Talislanta - but it is still a science fiction setting, and thus the question arises how to deal with science fiction technology while still evoking the general feel of Talislanta.
High tech and ultra tech devices can evoke a sense of wonder and thus have an important role to play in such a setting. But they shouldn't dominate the setting, either - or else the setting becomes about the latest toys and not about the scattered branches of humanity's descendants. There is no unified technological infrastructure - the various colony worlds had many, many centuries to diverge technologically as well as culturally, and even though travel between the worlds is once again possible, even traveling to fairly close worlds can take weeks and bulk trade in goods is tremendously expensive. And as there is no unified interstellar empire, no standards have developed.
Thus, while researchers and engineers from one high-tech world may understand the principles of devices from another and even eventually be able to replicate them, these devices won't necessarily work with their own gadgets - and the higher the tech level of the device, the more difficult it will be. Ammunition sizes will be different. Power cells will be different. Connecting plugs will be different. Programming languages will be different.
Thus, whenever the PCs acquire some type of high-tech gear, they should note the world of origin. When attempting to combine it with other equipment, it will only work automatically with devices from the same home world. It may be possible to make it work with devices from other worlds, but that might require days or even weeks of tinkering - which the PCs either need to do themselves, or pay someone else to do it. Attempting to fix a device with replacement parts from the wrong world will run into similar problems.
The goal here is not to deprive the PCs of all their toys, but encourage them to make use of more rugged low-tech devices as much as possible, apart from a few important "signature devices" where their benefits to the character outweigh the maintenance problems they will cause.
As for what kind of technology is available in the first place... well, in GURPS terms (which happens to be my favorite RPG), the most advanced worlds manage a solid Tech Level of 9, which is comparable to mid- to late 21st century. Some manage to be more advanced (Tech Level 10) in particular fields of technology, and a very few are able to create a limited number of Tech Level 11 items - but each high-tech world has its own specialties, and none is better at every field of technology than all the others.
But there is a wide range of Tech Levels. Those who meaningfully participate in interstellar trade tend to have at least Tech Level 5 (Industrial Revolution - remember, you don't need starships in this setting to travel from world to world...), and the planetary elite will accumulate more advanced technology to help maintain their rule. But there are worlds that have collapsed all the way back to the Stone Age, and anything in between. Of course, the lower tech worlds tend to attract the more adventurous outworlders, especially those who want to carve out their own emperors - but psionics may be the great equalizer in this.