What it asks. I do often enjoy the Imperium, but while there are plenty of stereotypical aliens around to fight, and the ever-growing marauding armies of Chaos, sometimes it could be interesting to have a second group of Humans around, who don't worship the Ruinous Powers (the Severan Dominate?), who are at least remotely close to being the Humans we might be, today, and who can make due, at least in part, without the Tech-Priests, the other Priests, and all the other organizations I've occasionally said "how do you still have these, now that you are Excommunicate Traitorus?" Hell, the "they have cool toys?" part of me might be satisfied just knowing that they have suits of power armor that make their warriors into centaurs, and with AP3 guns.

So, here's the thing. Not a lot known about this organization, which is fine, but the sources seem to claim that the entire group was wiped out before Horus even went evil (partly involved in Erebus' plot to taint the Warmaster). They are a group of people with Imperium-comparable ships, so they could have people anywhere, but they are all dead? This doesn't seem like something that would stop me, but I'm wondering, has anyone who likes the minutiae of 40K plots and lore enough to know of them actually used them (assuming you say "well, the Imperium thought that they were all gone, but they've thought a lot of crazy things, over the millennia)?

I'm not immediately planning to set up a remote combine of Interex factions, conveniently hiding out in the remote reaches of the Koronus Expanse/Jericho Reach, but if anyone else has used them, or a group like them, I'd be interested in hearing a bit about it. I have an inkling of what they were like in the Emperor's time, but many things change over 10,000 years; not sure how they might've, considering they'd have to of been hiding from the ever-growing-in-strength Imperium. If there can be so many Human colonies, even this far out, who have little to know memory of the Emperor, His Imperium, and all that jazz, it sort of stands to reason, at least to me, that somewhere among them might be the Interex, or a group of similar descent (Baseline Humans with more Tau-like technology, maybe even government idealogy; much like various examples of "Humanity" from numerous other sci-fi lines).

Not really the Interex, but a similar planet. Equivalent of an Imperial World, with a massive stash of Archeotech. No working STC, unfortunately, but still a solid base. The system has been isolated by one of the peripheral warp storms, having only been safe due to an effect similar to the Cadian Gate. Our Dynasty found them, realized that they would be declared heretics by any nearby Imperials with their ideas of 'democracy' and 'secular thinking' - so the Dynasty carefully eased them back into Imperial Baseline. We kept 'em in their own system with stories of a massive plague loose in the wider galaxy, and their planet was a pristine pearl in an otherwise nasty universe (true in a fashion). Made a tidy profit importing "stringently checked goods", ofc.

Imperialization process is still going on, but recently they wized up to the fact that there are other functioning Imperial Worlds out there, so they now want warp drives to explore the wider universe on their own. Let the good times roll...

they would be declared heretics by any nearby Imperials with their ideas of 'democracy' and 'secular thinking'

Just a bit of offtopic - generally, the Imperium cares not about the type of planetary government preferred by local inhabitants, and there are generally only two reasons the discovered non-Imperial world would be judged heretics on spot - they have to either worship Chaos by their own will or willingly cooperate (or worse, coexist) with xenos.

A democratic government, maybe if there are clear rules regarding the Imperial Tithe. Secular thinking, absolutely not. The Ecclesiarchy is not particulary known for allowing worlds to remain forsaken of the Emperor's Light - if initial Missionaries would be rebuffed the next wave would come with more pursuasive (and concussive) arguments.