The speculation is that those 3 were Legions definitely intended to persist after the Great Crusade (whether/how many of the others would is a different argument). You got it mostly right with the Wolves and AL. The Wolves being the larger scale security/threat enders/border patrols in a post xenos races and either bringing human-occupied planets into compliance or destroying them if they resisted.' class='bbc ipSeoAcronym'>GC Imperium (especially given their isolation on Fenris, they'd pretty much stay happily in the box until needed). The AL were probably envisioned as the space FBI, doing all the secret sneaky things, and if anything went wrong they still have the force of a Legion to smash the bad guys.
The Salamanders, remember, had a pretty major character change with the discovery of Vulkan. Before that, they were horrifically stubborn, with an almost macabre infatuation with taking on, and beating impossible odds. The best idea I've seen is that the XVIII were meant to be the Guardians of the Imperial webway. A very patient Legion that holds impossible odds as an aspirational goal? Sound like the perfect dudes for long garrison deployments, and if there were problems with the webway, they'd likely be able to at least hold their ground until the rest of the Imperial military (surviving Legions, Talons etc.) could mobilise and get to the crisis point.
Tbh, this is the best theory I think I've seen of the reasoning behind the Salamanders and their place in teh Trefoil.
The possibility I myself had considered back when their Black Book came out was that they were there for a sort of 'ideological' purpose. Except where the XVIIth were supposed ot burn down things that didn't 'fit' with the Imperial Creed, teh XVIII were supposed to more 'humanistically' put acrosss what the Imperium was supposed to be believing & thinking. With occasional more 'direct' purgation as and where required, but without the same crusading zeal of the Imperial Heralds. And, alongside this, a 'collect-and-confiscate' approach ot some of the stuff they were coming into contact with [kinda like, come ot think about it, how the XVth wound up working - as we see during the confrontation with the Yeselti in Prospero Burns as part of Unification]
Either that or, as you've proposed, the way they 'complete' the Trefoil is by providing more static, numerous and redoubtable bodies to go with the 'shock and aaargh' of the VIth, and the 'The Emperor Knows All, Sees All, Subverts All' vibe of the XXth. It's quite complementary, in that regard; and one could suppose that the XVIIIth's greater ability to relate to ordinary mortals might have made them superior long-term occupation/'peacekeeper' troops scattere throughout the Imperium in particular areas requiring a more 'long-term' ensurement of compliance/obedience [after all, the other legions which do this sort of thing - like the IVth - tned to become bitter, paranoid, and aloof from tehir mortal subjects rather than 'in your community' winning hearts and minds and such]
Vox Stellarum efforts:
The Unyielding Adamanticores
[truescale Astartes, auxilia, and sectorial AdMech; M37/38]
The Haunting Harii of Hvergelmir
[truescale Astartes ... and associates]
The Unification Wars
[truescale Thunder Warriors, (Proto-)Astartes, Army of Unification, Aegyptian-style Proto-Mechanicus raiders, etc.]
[Tomb Raiding for Fun & Prophet]
[truescale loyalist IVth, VIIIth; Agents of the Sigillite; VIth; and so much more; and Umbral's amazing Traitors]
The Worlds-Wide Webway
[(Dark) Eldar, Imperial Inquisition, an a-maze-ing realmweave of fear and wondermeant]
[some rather different Inquisitorial Storm Troopers, true-scale Fire Hawks, Black Dragon, Inquisitor and associates, etc.]
For Whom The Great Bell Tolls Thrice
[attempts to put the 'priest' back in 'tech-priest' with more-medieval mechanicus]
[the log that incepted it all - at least two Inquisitors, truescale Deathwatch, local enforcers, cultists, etc.]