Fit for tone and the rest of the system.
Pathfinder 2E sits comfortably in the heroic, high-fantasy niche. Character customization, mechanically speaking, can be fairly involved. Various factors make it so that even low-level characters have a variety of options to consider in combat, and a basic "swing as many times as possible" approach is rarely good; and it is intentionally designed to encourage teamwork in everything from positioning to debuffing to recalling and communicating knowledge and what-not. As one might expect from a heroic game, it is
not simulationist in terms of lasting wounds and what-not, and most damage won't actually impair you until it actually downs you; and most attacks also won't cause you any mental trauma of any kind. It's also designed so that the encounter building rules should
generally work decently in terms of estimating encounter challenge, between such things as limiting modifier stacking and attention being paid to how most character and monster features get balanced.
Twilight 2000 is designed to be a gritty survival game, where combat is generally going to mostly rely on firearms, vehicle cannon, and explosives. The 4E rules encourage such things as suppression and overwatch, careful use of cover, attacking from ambush... torso and head hits might kill instantly, and injuries anywhere can lead to death (on various timeframes -- as fast as checking every several-second round, as slow as bleeding to death over hours and requiring similarly involved surgery to 'fix') if not treated; eyes, arms and legs can be permanently lost. Even bullets that don't physically hit you, might inflict stress damage that can result in a lasting disorder. Combat is meant to be nervewracking and potentially very unfair, and should only be engaged in with great care along with ruthlessly seeking every possible advantage.
Call of Cthulhu is a lot less combat-focused than those games, and -- aside from the clunky rules for automatic fire, which personally I'd rather replace with mechanics from
Delta Green re: lethality rolls -- relatively streamlined. Even
Pulp characters won't have that mechanically-complex repertoires, unless they have sacrificed a
lot of sanity and time diving into grimoires and practicing rituals. "Theatre of the mind" is a common way of handling combat, as exact positioning is generally not that crucial. There can be
"nothing happens" turns, but hit point bloat doesn't exist (at least for investigators!) so combat remains tense as investigators are always under serious threat in any 'real' combat. Combat will tend to be resolved fairly quickly, perhaps by fleeing as quickly as possible

, with any survivors resuming (or terminating in fear) the rest of the investigation.
Paranoia XP is very fast and loose with the combat rules, with a lot of explicit room for GM fiat and high lethality. Troubleshooters are meant to be killed pretty easily and just as easily replaced by clones. There's no real tactical crunch. As a satirical game with a very specific tone, death is meant to be fast and funny, and you really aren't meant to be doing things like carefully considering plans of attack and optimal use of whatever abilities and gear you have -- if you even
know for sure what gear you have, since maybe you've been ordered to test conveniently unmarked and unidentified experimental gear or w/e.
All four of these have combat systems that reflect the purpose of play, and they make sense for the sorts of games they're trying to be. Swapping those styles out would work less well. My personal inclinations run towards the more serious end and I'm perfectly fine with crunchy combat systems, but, say, I wouldn't encourage trying to shoehorn TW2K4E's system into something like
Paranoia XP; it just doesn't seem appropriate; nor would I suggest replacing
Pathfinder 2E combat mechanics with those from
RuneQuest, which are more appropriate for rather fewer but much more hazardous combats.