Verisimlitude is the quality of seeming to conform to the norms of reality. Put simply, it is the quality where a fictional context nonetheless is--barring explicit callouts, such as supernatural power or hypothetical technology--things that happen in that context conform (more or less) to the way they do in the real world. Gravity works as expected, injury and recovery works as expected, computers work as expected, societies work as one expects and so on.
With regards to role-playing games, this is a tricky thing. The first problem is that there is, alas, a wide variation in the knowledge base of the gamer population. Ignorance of basic facts in computer science, ballistics, metallurgy, psychology, and many other arts and sciences with direct and immediate application to the common scenarios and experiences found in RIFTS make expectations of outcomes and consequences differ a lot and as a result what an individual believes is a norm of reality is not necessary what is a norm. While this nescience and ignorance can be remedied--and I urge you to do so--what this means for your table is that maintaining verisimlitude means dealing with a moving target.
The remedy is to fix the target. You fix the target by laying out, in clear and explicit terms, what a player can expect at your table. While that may seem silly in the context of RIFTS, it is not; by establishing a clear and explicit set of expectations you also establish a basis for all of the house rules that you issue which address concerns of verisimlitude, which makes it far more likely that players will accept your rules with no complaints. By showing your logic, you make buy-in easier and therefore avoid needless drama.
So, let's talk about a common issue in verisimlitude: getting hurt.
The game, as most tabletop RPGs do, presumes that a character is Human or something close enough for Human norms to apply. This means that the game assumes that Human norms of physiology and psychology also apply by default, which means that it assumes that what is normal for Humans when they get thrown off cliffs, shot, stabbed, etc. applies to near-Humans as well (with some variances).
First, now that YouTube is a thing, I recommend that you go there and watch as many videos as you can regarding the testing of ballistic armor and contemporary ammunition as you can find- especially those that take the time to explain terminology or link to definitions, as you're likely to be new to the topic and thus not up on the lingo. Then do the same for pre-modern arms and armor testing, videos on injury and recovery, etc. You will benefit in your everyday life, as well as at the table, by acquiring basic familiarity with how the body reacts to injurious force and how it recovers from it. (Yes, you're likely to hear a lot of opinions you may not like; that's the price of your self-education.) This has a point: to inform you as to what the norms of the real world are about the subject, which gives you a foundation for sound rulings thereafter.
Second, take a little time to look over how the Palladium ruleset operates. Think of it as a machine or a program, and you're inspecting how the parts or the code interacts with itself as it operates. You want to create a ruling that doesn't unduly complicate or monkeywrench the machine, so some active engagement with the mathematics (such as it is) of Palladium's game engine goes a long way towards creating a ruling that not only will players not find objectionable but stands up to the rigors of actual play.
Finally, do two series of tests. The first is your "white room" testing that you see often online in gaming forum discussions. These focus on the mathematics and ensure that the mechanics work as intended. The second is actual play testing, which ensures that the rules function in the field (as it were) as intended. Repeat until you achieve the desired results.
So, let's talk injury and recovery.
The basic paradigm is sound. Some injuries merely bruise, and while painful they are not necessarily serious and recovery from them goes quickly compared to those that draw blood or damage bone. This is what S.D.C. is intended to handle, and so long as that premise is kept in mind it will work. Serious injuries are what Hit Points represent. (Compare with Stun and Hits in HERO, or Bashing and Lethal in Exalted.) Massive damage of a nature inherently beyond what the body can handle is a matter of scale, which is where Mega-Damage works as intended. It is only execution that is lacking.
So, let's try a rule: "All kinetic damage stopped by personal armor also inflicts equal damage to the target's S.D.C. score." The intention is for damage sources relying upon kinetic force for its damage--firearms, primarily, in a RIFTS context, and similar weapons such as railguns and explosives relying on concussive force or shrapnel--to be able to injure a target even if it is suitably armored. It produces play results suitably similiar to real life: someone wearing personal armor who gets shot is still likely to be hurt, and even taken out of the fight, but will not only survive the hit but be back in action in days if not hours.
The cost here is that you need to define what weapons or powers are subject to this rule, and then to what degree. You need not do this up front; you can--and if you are in a reasonable group of people, you should--make this decision on the spot as required. Take your knowledge and apply it as you go. The tabletop role-playing game medium cannot work as intended any other way.