Only CT, MT, and MGT are inter-compatible to each other, and even then, only characters really are.
T4 and T5 may be, but T5 CGen is not yet fixed to stable. At present, the draft T5 produces characters somewhat compatible with T4.
Classic Traveller: baseline. 1-6 skills per term (1-2 Term «extra on 1st term», 0-1 Commission, 0-1 Promotion, 0-1 Service, 0-1 Rank, ), average about 1.6, stats 2d6. Attribute raises cost 1 skill level.
Generally, most characters have a level 2 skill or 2, and a lot of level 1's, tho a single skill at 3 or 4 isn't actually rare.
Mega Traveller: 1-8 skill levels per term (1-2 Term, 0-1 Position, 0-1 Promotion, 0-1 0-3 Special Duty «bonus possible on Position, promotion, special duty», 0-1 Service skill, 0-1 Rank skill), average about 2.5. Stats 2d6. Attribute raises cost 1 skill level.
Average skill level per skill is about the same as CT, due to more non-weapon skills; due to many more pick-from-list skill receipts, it's possible to get skills up to level 6 for many starting characters.
Traveller: The New Era: 1-8 skills per term (1-4 term, 1 Hobby, 0-1 Comission, 0-1 promotion, 0-1 Special Duty); Term skills per term starts at 4, and drops with age. Attributes 2d6-1, stat gains cost 2 terms' hobby choices.
Conversion from CT/MT in back of Survival Margin
No conversion needed for Twilight 2000 Second Edition nor Dark Conspiracy, but note that those always give 4 skills per term.
T4 aka Mark Miller's Traveller: 4-9 skills per term. (4 for term, 0-1 Commission, 0-1 Promotion, 0-1 special duty, 0-1 service skills, 0-1 rank skills). Attributes 2d6, attribute raises cost same as 1 skill level.
Note that skills are much cheaper than CT/MT; younger characters on par with TNE characters except for attributes.
Mongoose Traveller, aka MGT: 1-5 skills per term (1 term skill, 0-1 from events, 0-1 promotion, 0-1 rank, 0-1 service skills), averaging about 2 skills per term, plus 1-3 skills per character at end of generation (0-2 from connections, 1 from party package) plus lots of level 0 skills (5x level 0 skills in first term, 0-5 background level 0 skills). Attributes 2d6. Stat raises 1 skill level each.
Most characters will have lots of level 1 skills; a smattering of 2's and 3's. Due to a different skill list, compatibility with CT and MT is less than 1:1, but MGT and MT characters in the same party are not a terribly big issue.
GURPS Traveller: Stats purchased in range of (typically) 7-18, but readable as if generated on 3d6 or 2d6+6. (IE: D&D kind of range, rather than CT's 2d6.) Skills bought with points. Totally incompatible characters.
T20 Traveller's Handbook: Characters are comparable with D&D 3.0. Totally incompatible characters.
Hero Traveller: Characters are standard Hero System characters, with Traveller specific setting packages.
CT has two systems: Bk2 and Bk5
MT has one.
TNE has two fully compatible: FF&S1 and Brilliant Lances.
T4 has several: QSDS, SSDS, FF&S2; all are compatible.
MGT has two: core and MGT Bk 2.
T20 has one, and it's a CT Bk5 derivative.
GT has at least two, plus GURPS Vehicles, from which both were derived.
HT has one, it's a CT Bk 5 derivative.
T5 at present draft has 2 planned, and one released to playtest.
CT Bk 2 is similar to MGT core for design, but combat is very different.
CT Bk 5, T20, and HT designs are all close enough to simply look-up and rerate for the new system.
MT's USP ratings are scaled identically to CT Bk 5, but the underlying design system is incompatible. This means one can easily use CT Bk5 and T20 designs in MGT by ignoring the design rules, and just knowing how to read the ratings. And vice versa. MT's ship combat rules are literally just a task-system adaptation of CT Bk5's combat system.
TNE and T4 use the same ship and vehicle design rules; FF&S2 includes rating for T4, and extra options. FF&S 1 and BL include rating for TNE. SSDS and QSDS are built from FF&S1, producing legal but non-optimized designs, and rate for T4.
MGT core is similar to CT Bk2, and CT Bk 2 designs usually come across pretty well, but need to be rerated, and often have surplus space left if redesigned.
MGT Bk2 is designed for larger ships than MGT Core allows, and produces MGT Core compatible designs, but they are incompatible with other editions.
T5 ACS is a system unique to itself. It resembles slightly CT Bk 2 and MGT. The other generation rules, not yet in playtest, will be for large ships.
CT: No ground vehicle design rules, per se... the minis game, Striker, is CT compatible, and includes design rules, so if one swaps out Bk1 combat for striker, striker vehicles can be used. (Azhanti High Lightning is a boardgame version of the striker combat mechanics)
MT: Striker designs can be used with mostly the same ratings, but hits need to be figured from the design. MT designs include everything needed to drop them into Striker, however...
TNE: FF&S1 covers vehicle designs. FF&S 1 designs can be rerated for T4
T4: FF&S2 covers vehicle designs. FF&S 2 designs can be rerated for TNE
T4 Emperor's Vehicles has an alternate vehicle design system rating only for T4. It doesn't convert to TNE well.
Except for GT, they all use the same system generation rules, with very slight variations, so all mapping and world data for the non-GURPS versions is compatible.
Combat stats and mechanics are so different that simple crossover isn't readily practical; one can, however, pretty much pick one's choice of CT Bk1, Striker/AHL, MT, MGT or T4 combat and use it with the others. There are some hiccups: CT has no task system, and thus, due to skill level differences, T4 isn't really compatible with CT Bk1 combat, and you need to borrow a task system to use any non-CT combat system with CT.
Setting Information - except for MGT and CT, the setting is built into the core books to some degree. CT, it's in the expansions (Books 4-8; Supplements 3, 4, 5, 8, 10, 11, and to a lesser extent, 12 and 13; Adventures 2-3 & 6-12). MGT, it's in a separate "Third Imperium" trade dress supplement line.