You ask 'where did I get that from' concerning 'Orc-Captains' being 'Maiar'?
Didn't you read the quote I posted?
Tolkien states it plainly:
'The least (of the these corrupted spirits, like Sauron & Balrogs) could have been primitive (and much more powerful & perilous) Orcs.
Oh, here's the quote on reproduction. It follows immediately (I'm looking through all the footnotes at 6AM, lol)
'But by practising procreation they would (cf. Melian) [become] more & more earthbound, unable to return to spirit-state (even demon-form) until released by death (killing) and they would dwindle in force.
Now what was that about Melian not applying here? [

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In case you're wondering, All this is page 410, HoME, Volume 10; Morgoth's Ring.
Finally, I believe you're on the wrong track about Sauron & Balrogs being 'relatively equal' because they're both Maiar to start with...
Tolkien states plainly that they're not. Shouldn't HIS word be our final authority ~ I know, what a crazy notion these daze! [sly]
seriously though, we might as well say by this line of thinking ~ 'they're both Maiar', we can carry it a step further & say: 'Balrogs & Valar are equal because they're all Ainur'. ~ humm...
See the slppery slope you've taken?
The fact is, Tolkien ~ not us ~ has clearly laid out that these spiritual beings, which come from the same source, have a huge range of 'spiritual power'.
In fact, by the time Malkor dwindles in strength to become Morgoth (BIG difference), he no longer even has the strength to take on certain Maiar who serve the Valar, like Arien, or for that matter, in The War of Wrath, Eonwe too.
BTW: Thanks for the greeting! [cool]