I still have my copy of Clan War.
Same. For 2nd or 3rd (and presumably 4th, tho' I skipped 4th other than core; I was happy enough with 3) , it's a great adjunct.
It's less useful with 5th, due to different assumptions, but still, PC's it's just use their rings as is for their CW ratings.
I never got much into the card game, and my kids destroyed my Diskwars disks.
Which reminds me...
The interlocking systems idea...
TSR, GDW, FASA, WEG, & DP9 all had games at various scales for some of their major game lines. AD&D
Battlesystem.
Traveller's
Mayday,
Snapshot,
Azhanti High Lightning,
Imperium,
Dark Nebula,
Fifth Frontier war,
Striker. of those, Imperium, DN, & FFW were not readily useful in most campaign styles
FASA Battletech (too many to list)
FASA Renegade Legion:
Interceptor,
Centurion,
Leviathon,
Circus Imperium,
Legionnaire. Legionnaire is the RPG.
FASA STRPG: ST3 Ship Combat game (expanded from the core RPG rules in STRPG1e)
WEG: Star Wars, Star Warriors, Star Wars Miniatures Battles (and 3 not readily used in RPG board wargames I'm too lazy to look up)
DP9: Heavy Gear: the RPG and the Minis Game use the same rules. Some editions, they were a combined rulebook.
DP9: Jovian Chronicles: same system as HG, and likewise, some editions combined the minis and RPG rules in one volume.
I wonder if we'll see more like those.