This is all about how thousands work in Latin.
The singular mille is an undeclinable adjective, the plural milia is a third declension noun.
When you are counting the youth, with mille the young are in whatever case you need and with milia it is milia that is in the correct grammatical case and the young are in the genitive.
In the singular the number is the ...
As...so... can be represented in a few ways, but I like tam...quam... for it:
tam de superiis, quam de inferiis.
Just as from above, so too from below.
This also gets to the Arabic better, if the Wikipedia article is accurate.
The phrase you are looking for is either of these:
Ut supra, sic infra.
Ut supra, ita infra.
They both mean the same thing. I think the first one sounds better.
I just searched a bit for these on Google Books and was surprised not to find many hits. I thought I'd first encountered this phrase in descriptions of the medieval worldview, as a well known way ...
I agree with all Hugh's comments, but there are a few other small points in your translation that could be changed a bit. Here's a full translation:
Octavius Durantes, distinguished in humane and theological subjects, labored with extraordinary skill and in most beautiful [style] on volumes by which he rendered himself useful to his contemporaries and ...
Is it posible that Quibus are the Volumina 'by means of which (volumes)' ?
and the other two would then be datives "by means of which he rendered himself useful to the present (generation) and well-known to the those about to be."
Ainsworth in the classical section gives scloppus, or sclopus as 'a sound made with puffing of the cheeks'