So I've done my research on them and one thing keeps striking me; why are they so limited? You think them being such a huge aspect especially since it's a military branch, there would be more variety for them. I'd think they'd have some form of actual tank, maybe a variant of Leman Russ tank or something but they don't, which is just bizarre... To me, anyways.
The Exorcist is pretty damn close. Whilst it looks like it should be an artillery vehicle like a whirlwind, it has heavy frontal armour and it's a decent match for the Astartes Predator.
Remember that the Adepta Sororitas aren't exactly a front-line military; I mean - they are, and one of the most elite ones in the Imperium, too. But they rarely go off to full-scale war by themselves. Internal purges, yes, of a world's population or even (when they're feeling confident) of an astartes chapter. Or accompanying a War of Faith.
But because they only really exist as a legal 'cheat' to allow an organisation sworn not to maintain a standing army to maintain a standing army, it's in the Ecclesiarchy's interest not to draw attention to this by equipping them beyond their traditional capabilities.
The Sororitas' wargear is just as ritually significant - probably more so - than the equivalent wargear in Astartes service. They don't just grab extra stuff and paint a fleur-
de-lys on the side of it. If they need long range siege artillery, or big-ass battleships, or whatever, you'll generally find it provided by the guard, or the navy, or the Inquisition (directly or indirectly).
There's nothing wrong with a guard regiment (or tempestus regiment or whatever) being assigned to support them.
Is there any information lingering about that properly describes the numbers an ordinary Order Militant usually has? I know there are different Shrine Worlds and other related places where one to a few hundred sisters will protect but a definitive number would be fantastic!
Regarding Orders Non-Militant, I've noticed there's quite a bit that differ immensely; sometimes it's like night and day. Would it be out of the ordinary for a sector, with a rather imposing Ecclisiarichal presence, to have ten, maybe even twenty Orders Non-Militant?
Depends on the sector, but generally you'll find that only one or two orders turn up in a given easily-traversable region (like a sector). There is no maximum size for an order - minor orders may only be a single convent somewhere, but the bigger orders simply create a new preceptor or commandery alongside the existing ones whenever you get a new area of responsibility assigned to them.
The best example is (unsurprisingly) the Order Of Our Martyred Lady - it's the only one we have any detailed novels covering - Fire & Faith and Hammer & Anvil.
A Major Order, it's based at Ophelia (along with two others). But it has enough members to field an army capable of taking a fortified city on Neva (admittedly an important cardinal world), plus enough to re-occupy Sanctuary 101 (approximately a short company by astartes or guard terminology) from far enough away that they didn't really know what happened. A major order is likely to have dozens or more preceptories across the galaxy - the equivalent of several guard regiments of troops.
How many in a sector? Depends. The 'biggest' recognised subdivision is Preceptory, which is implied to be multiple company equivalents (as the Commandery is a company-analogue).
In Legion of the Damned, a cemetery world has a couple of dozen. Vraks had the same. You'll probably rarely find a world with a 'cardinal' without a sororitas mission, but hundreds or more will only occur where there's something really important - a sector cathedral with dozens of saints and the sector synod, for example, or a critical nexus of pilgrim routes.
Disposable abhuman regiments are definitely a thing. As noted, suicidal beastman units are a good way to buy you time to extract units you give a damn about from the battlefield.