I’m going to start at the bottom and work up, because I’m like that.

Rules generally get their free release a month after the retail launch of the game, I expect this to be the same for Firefight.

Mantic have said that they will not be releasing an UE style book but they do have plans for new factions in the future, rebs, goblins, nameless, and a secret faction they’ve not mentioned yet.

Fitting 40K armies into Firefight is actually quite tricky, Mantic’s universe is more rounded than 40K in some ways being more hard sci-fi than space fantasy.

The seven factions that will be in the Firefight Rule Book are

1. Enforcers
2. Forge Fathers
3. Marauders
4. GCPS
5. Plague
6. Veer Myn
7. Asterians

Enforcers and Forge Fathers would both work for the power armoured troops.

Marauders are orcs so fairly easy to map.

GCPS Imperial Guard

That ends the easy ones

Plague, would actually work for 40K orcs again, but also for Tyranids, you could run them as demons there are melee only options like zombies, stage 2a leapers, dogs and murderbirds.

Veer Myn again could be orcs again.

Asterians, yes they are space elves but when you’re a dying race you don’t go out in the field you send 1 guy to run a hundred robots. So yes you can run them as eldar but actually you could fit necrons in there as well, or indeed mix them.

Mantic don’t do ridiculous sized kits so all the overpriced nonsense will have to stay on the shelf, sorry.

No alliances, you could house rule it but you’ll need to work out how you’re going to deal with command points and structure

Class                                    Typical Base Size                Typical Number of Models in a unit
Troop                                      25mm round                                        Five
Specialist                               40mm round                                        Two
Support                                  60mm round                                        One

that’s the basics for the types of unit and base sizes you’ll be dealing with. I honestly have no clue whether 40K models will line up, I stopped paying attention to what GW is shitting out a long time ago. I could probably stick stuff into the lists and it would kind of work, but despite what some GW fanboys think Mantic isn’t just copying GW and Firefight and Warpath don’t easily mirror 40K

Even Kings didn’t match whamster that well hence having to write a book with 9 lists in it out of the 11 armies that existed in the old world (or 14 if you count all the elves separately)

If the appeal is playing a better/quicker game with current 40K armies I would suggest just going to One Page Rules and playing Grim Dark. It has analogues for all the 40K armies. If you want to play firefight you’re better off just playing firefight with the mantic models, or you could do that 3D thing the kids like. A lot of GW figures will be redundant in FF

If I had to try and map Warpath to 40k, I would probably say that I would likely use the following lists:

Enforcers – Space Marines

Marauders – Orks

GCPS – Imperial Guard

Asterians – Eldar, possibly, sort of

Plague – No direct match

Veermyn – No direct match

Plague – No direct match

Forge Fathers – no direct match

The lists with no direct matches doesn’t mean you can’t use them.  Like Gerry said, there’s scope for just proxying in.  So veer-myn could easily be used for Genestealer Cults, as could plague.  Necrons could use the Asterians list etc.  Forge Fathers could also be used to represent both Imperial Guard and Space Marines having a mixture if heavily power armoured troops and light to medium.  You might have to get creative though.  Also worth noting that firefight isn’t particularly vehicle heavy.  They exist but you wouldn’t expect to see them in the same numbers ot with the same focus as you see in 40k.

I think this is almost a testament to how far Mantic have come, moving out of the shadow of GW and establishing their own world with their own ideas.

It’s a shame people get stuck in the GW rut and don’t seem to want to dip their toe in the pool of other wargames.  Despite still not having rebs (by far the most numerous faction in my collection) I’m sacking off all my Imperial Guard Miniatures (I say “all”, like two squads) and expanding on my GCPS Deadzone stuff for Firefight.  It’s cheap enough that you can do that really, you won’t spend an unreasonable amount of money on Firefight to get to the “standard” size games.  I have the old GCPS Faction booster and starter from the previous edition of Deadzone and a squad from the new version.  I don’t actually need to add a great deal to that, maybe one more squad and a vehicle, to have the equivalent of the Firefight starter box.  I hope you can get your gaming group to maybe give Firefight a try, even using 40k proxies, because while Mantic’s Miniatures haven’t always been the best, their rules are almost always great fun.

If you are just trying to stir up some interest, then it doesn’t matter what models your friends use as long as they are engaged with the ruleset and enjoying themselves.

Let them use any model they want. Who really cares?  You do not want people experiencing a new game system with armies based on what is asthetically close to a 40k army.  That is extremely lame, and literally has nothing to do with how an army plays in Firefight.

As for which force/army lists they should use that should match their favored play style.   Do they want to play more horde type armies (Veermyn/Plague for example) or more elite forces (like Enforcers or Asterians).  Or have them do some online research and make a choice based on fluff and backstory information.  Those two ways of picking an army will have them far far more engaged in game them, then basing thier choices on 40k comparisons.