I figured I'd try to answer the part of the question that most of the other answers haven't really touched on yet, namely "How do I decide whether a particular piece of tech would be too disruptive?"
One way to approach this question is to ask yourself, "How could the players achieve the same effect using things that are listed in the books?" If all the effects of the tech you're considering could already be achieved by other means (even if they might not be quite as practical), it's unlikely to completely break the setting. It might still have an effect on game balance, of course, but you'll at least have some kind of upper bound on how much difference the new tech could make by comparing the difference between the new and the established ways of doing things.
Taking your portable medkit for synthmorphs as an example, you should ask yourself "How else could the players heal a sick or injured synthmorph?" Maybe they can take him/her/it to a hospital; a hospital isn't portable, but it means that all the players are saving by using the medkit is the time and effort to travel to the nearest one. That could still make a big difference, depending on just how far the closest hospital is, but at least it gives you some idea of how big the difference could be. Or maybe your setting has magic as well as tech, and there's already a healing spell that works on synthmorphs, in which case having the medkit just means the players don't necessarily need to bring a mage with healing skills along.
Of course, when using this method, you should generally try to keep the effects of the new tech fairly conservative compared to what already exists in the game. For example, the medkit probably shouldn't be able to fix anything the hospital couldn't. In fact, its abilities should most likely be strictly inferior by a considerable margin, both for the sake of realism and to offset its convenience. Also, since the players can't really keep running to a hospital every few minutes, there should probably also be a limit on how often the medkit can be used, and/or on how long it takes to do its job. And, of course, if the setting clearly implies that something is generally lethal or disabling to synthmorphs, well, the medkit probably shouldn't be able to fix that.
This conveniently segues into another question you can ask yourself, namely "What effects would the existence of this tech have on established (or planned) elements of the setting?" For instance, try to think of any events in the backstory of the setting where having this tech might've made a difference. If you find yourself thinking "Well, those guys sure wouldn't have lost that battle if they'd had these medkits," and you can't think of any good reason why they shouldn't have had them if they existed, that could be an indication that adding that tech to the setting might at least disrupt the consistency of the backstory, if nothing else.
Also, think not just about what your players could or would do with the tech, but also about what their enemies (and other NPCs) could do with it. You're going to need to think about that sooner or later anyway, unless there's some good reason why only the players should have it, so you might as well think about it before you decide that it really exists.
If you have a suitably devious mind, you should also ask yourself "How could I abuse this tech if I wanted to?" Obviously, that means not restricting yourself to how you or the players think the tech should be used, but just looking at the stats as you've written them and thinking "OK, if I wanted to min-max and exploit the hell out of this thing, and had enough resources to pull it off, what could I do?" OK, so one portable medkit seems pretty harmless; what if you had a hundred of them, and combined them with all the most exploity features already in the game, what could you do then?
In particular, beware of anything without limits. A box that can unfold to twice its size is probably harmless. A box that can keep exponentially unfolding forever could easily break the game.
If you're not feeling so devious, a simpler alternative can be to just ask your players up front what they want to do with the tech. Then, if it sounds reasonable to you, write the specs so that it does that and nothing (too much) more. One advantage of this method is that you can harness your players' creativity in finding ways in which the new tech could be troublesome. (In particular, the kind of people who like to come up with exploits for game mechanics are also typically happy to point out those exploits before the mechanics are made part of the game, just as long as they get the chance to demonstrate their cleverness.) It also, in effect, binds the players into an unspoken promise not to step significantly beyond the limits they themselves set, or at least gives you and excuse to step in and say "Hey, wait a minute, that wasn't the way it was supposed to work!" if they do.
Finally, if you're not sure, don't be afraid of saying "OK, let's try it and see." Playtesting is an essential part of balancing the rules of any game, and there's no reason why you can't do that with your house rules too. Just tell your players that they can have the tech for one session to begin with, but that, if it unbalances the game too much, it's going to go away (or be redesigned) for the next session. In extreme cases, you can even tell your players "OK, that was just way too broken, let's just start over and replay the session without it." As long as the players knew in advance that this was going to be a test session, and as long as they can agree that the test didn't work out the way it was supposed to, they'll understand that.