An easy way for a Game Master to take what is written and make it yours is to curate and shape the setting material to your liking. This is a liminal conceptual space where rules and setting blend, so you might as well think of this as an aspect to making House Rules.
To illustrate the point, I'll go with the Coalition States and its primary socio-economic institution: the Army. The Army is where the Coalition puts the able-bodied men within its reach. By putting all adult men into the Army, it subjects them to a single comprehensive authority with total control over their lives, so we'll do that and remove all civilian life from the Coalition States: the Coalition doesn't have an Army, it IS the Army. The rules effect? All Coalition characters are Male and Military. (With two exceptions, as noted below.) This culls the playable options down to a handful of Occupations (which, really, are military specializations) and exactly one race/sex combination; it also narrows the range of playable spaces down to those specific to military operations- something that an expansionist, manaphobic, xenophobic, racist, sexist, authoritarian, and totalitarian military dictatorship operating in a milieu that justifies its propaganda on an hourly basis would be.
(Exceptions: Dog Boys and Honeypots. The former are still male-only, but male Canine; the latter is where the very few women allowed outside the home operate, both in counter-intelligence and in foreign intelligence. You expected post-apocalyptic Nazis to do otherwise? Learn your history; they would, because in many respects they did.)
Let's get more specific now.
A Coalition man, regardless of his specialization, must complete Basic Training. This includes, since the technology exists, the mandatory implantation of certain cybertechnologies. Ear modifications that act exactly as real-world electronic ear protection headphones, eye modifications that guarantee 20/20 vision as well as instant polarization (making eye glasses/sunglasses pointless), lung modifications to ensure endurance in the field, and a datajack in the base of the neck- all imposed to get any given recruit up to baseline functionality at best possible speed. (Yes, the datajack is that important; it's easier to have a unit jack into a cybernetic network to get trained/briefed/etc. than to talk it out (since all Coalition personnel are totally illiterate, so written stuff is out).
So, rules: instead of a Coalition character having a choice, they get a set package of cybernetics as a standard feature of that Occupation, to be considered part of Basic Training and not Specialization training.
Now, let's do some variation thinking.
Abolish man-sized energy weapons and instead replace them with latter-day (but otherwise conventional) firearms. As our Coalition is based in North America, they will use foreseeable iterations of U.S. Army small arms or rivals thereof. Instead of the M9 (i.e. a Beretta 92 model), we'll grant the Coalition officers a latter-generation Glock 17 or 19. (Pilots or similar personnel will be issues a Glock 26, while Technicians will instead be issued a hypothetical Glock carbine--as a real one does not exist as of this post; other manufacturers do this--chambered for the same handgun cartridge, and thus uses the same magazines; we can base this on the Keltec Sub-2000, the Kriss Vector, or some other real-world model but I think just imagining an up-sized Glock filling out the same profile as the venerable M1 Carbine would do.) This will have some cascading effects, but I think those to be desirable ones that are beneficial to the game experience anyway.
Likewise, the standard rifle will be iterations and variations on the AR-15 platform (currently the M4 variation in the U.S. Army). Maybe the standard cartridge will shift to the now-new .300 Blackout cartridge, instead of the current standard 5.56x45mm NATO cartridge. Upgunned versions chambered for the .308 Winchester cartridge ("AR-10" sorts) may be the standard issue instead, depending upon the commonly-encountered opposition of the Coalition. Specialized operators will still use bolt-action rifles, often chambered for that Winchester cartridge by default; far bigger and more powerful cartridges (such as the .50 BMG) will be employed as required (and yes, for those not paying attention, such rifle platforms exist and have for some time).
The game's "Mega-Damage" conceit, when confined to its original scope of use (i.e. tanks and similar objects), works fine; the vast majority of entities in a RIFTS campaign do not qualify for such a status and therefore should be handled accordingly. (You can achieve much of what is implied by using Mega-Damage by other means, especially for super-natural entities, such as Immunities and Resistances or some other concept that other tabletop RPG engines handle very well.)
The rules effect? To eliminiate needless redundancy in conceptual spaces, as well as conceptual confusion, by keeping the baseline of Coalition campaigns as close to that of "The Army vs. The Monsters" as possible- and thus making buy-in very easy for new and returning players, since the unrealities they need to deal with are confined to a few Gee-Whiz widgets and the inhuman presences that they confront on a regular basis.
There, a set of examples of how you can make the body of RIFTS into the game you want- all at varying degrees of emphasis (rules vs. setting), degrees of player-vs.-GM engagement, degrees of in-play vs. away-from-table engagement, etc. so you can see what I intended to do to achieve the desired effect- and thereby do this sort of thing for yourself. This is one of the many reasons for why being as skilled and as knowledgeable of things beyond gaming matters; you can't do this well, if at all, if you refuse to improve yourselves as individuals.