@Brian Van De Walker - A lot is going to depend on where you date the end of the era of "Imperial Conquests", and, as always, a lot also depends on how sensitive you and your gaming group are to weathering the unbearable calamaties of a little anachronism here and there.
And it sounds like we're talking about a conversation where both the WWI kits and the Napoleonic British Riflemen are on the menu, alongside the zombies, werewolves, and a dash of steampunk.
To that party, the Italians are bringing Carcano rifles - a product of the Victorian Era - some uniforms that (to an untrained eye) wouldn't look very far out of place a couple decades ahead of WWII, and some really snazzy Alpine caps and pith helmets with outrageous feathers that would ook great on a classic pulp-era sci-fi/fantasy/horror/adventure tabletop.
And, is there a better match anywhere for Italians from anywhere between the 1890s and the start of WWII (when the classic age of "imperial Conquests" was basically punctuated by the atom bomb?)
Now, if you're strictly looking toward the earlier end of the rough boundaries for this era, toward the Napoleonic era (and earlier - I'd argue that it surely began with the advanced colonization of the Americas, before the American Revolution!), then it's a different story. The wider you cast this net, the harder it's going to be to ignore the anachronisms - those Italians are going to look a little more natural alongside the WWI British, Russians, Germans, and French, than they will alongside the Napoleonic British Riflemen.
I'd be fine with mixing Napoleonic and WWI troops on the same tabletop - improvise the best you can with what you've got to work with! - but, even I couldn't blame anyone for feeling a little anxious about immersion past a certain point.
I know the original topic specified Wargames Atlantic sets in particular, but if you're looking at a "pulp era" game setting with zombies and werewolves, I say those Italians will fit in well enough there. If you want to go full-steam Weird Tales, you've still got a few nice optons in 28mm plastic out there outside of Wargame Atlantic to work with, such as the Reaper Bones "Chronoscope", "Mythos" and "Deadlands Noir" figures, which include some great Cthulhu monsters and investigators, Victorians, steampunk characters, Weird Western gunslingers, and more... Reaper's fantasy range includes gargoyles, vampires, mummies, and all sorts of other useful gothic horror figures that would fit right in, too. There's also the Dead Man's Hand hard plastic gunslinger kits, and the Frostgrave cultists and wizards hard plastic kits, which make some great Cthulhu cultists, villains, sidekicks, investigators, and heroes - these figures tend to run a bit short and pudgy compared to Wargames Atlantic, but you can usually trade arms between these guys and Wargames Atlantic sets, and with a little work the heads can be modified to fit as well. There's also the occasional hard plastic set from one of Wargames Atlantic's other competitors aimed at the general Victorian Era - specifically the Zulu wars and American Civil War - which are also loosely compatible with perhaps a little modificaiton for head swaps, and might work within the earlier years of the age of "Imperial Conquests" - gunslingers, ACW soldiers, Victorian British soldiers, and the like will fit in nicely with various flavors of steampunk with no modification, and if you use the Wargames Atlantic Death Fields Accessory Sprue with those sorts of figures, you can add just that little dash more "punk" sci-fi to them!!