As you say, maybe I could just convert some spare Oathmark goblins to Uruks with a different paint scheme and 'Isengard-issue' wargear (straight-swords, round shields)...
Yes - the Oathmark goblins are definitely "goblin-soldiers of greater stature", given that goblin-soldiers of lesser stature appear to have been Hobbit-sized. I realise that I jumped the gun in that thread by saying that the Isengarders had round shields; I don't think that's ever actually stated. So the Oathmark shields are fine as they are, as long as they get a White Hand!
The reason I was originally looking for other figures for my Uruks was because I wanted more visual difference between them and 'lesser' orcs on the tabletop- I don't have my books to hand, but were there any 'lesser' orc infantry present at the battle? Because if not then that solves the issue for me!
You could argue it either way, but I don't think there were. There's a description of "Orcs" and a description of "fierce Uruks" - but Tolkien almost always describes the Uruk-hai simply as "Orcs" throughout the narrative of The Lord of the Rings. So there are no real grounds for reading "Orcs" as implying "not Uruks". I think the description of "fierce Uruks trained to move at great speeds", etc. is a kind of "in focus" description or close-up.
Why? Well, a careful reading of The Lord of the Rings indicates that all (or virtually all) of the goblin-soldiers of Isengard are Uruks. If you start with the earliest descriptions of them at Amon Hen and read right the way through to the Appendices, there's no sign of lesser orcs barring (perhaps) Ugluk's reference to a scout as "Snaga". But that scout might well be a Northerner (Ugluk praises their ability to see in the dark).
At Helm's Deep Gamling warns Aragorn that the "creatures of Isengard" will not quail at the sun - which hardly suggests that there are troops of sunlight-averse smaller orcs around. Also, a distinction is made between two shapes of enemy: "tall and grim" and "squat and broad". And in 'Flotsam and Jetsam', when Merry and Pippin describe the armies of Isengard marching out, they also make a binary description between Orcs and Man-sized creatures:
"Then all at once there was a tremendous stir. Trumpets blared and the walls of Isengard echoed.... All Saruman's people were marching away. I don't know much about this war, or about the Horsemen of Rohan, but Saruman seems to have meant to finish off the king and all his men with one final blow. He emptied Isengard. I saw the enemy go: endless lines of marching Orcs; and troops of them mounted on great wolves. And there were battalions of Men, too. Many of them carried torches, and... I could see their faces. Most of them were ordinary men, rather tall and dark-haired, and grim but not particularly evil-looking. But there were some others that were horrible: man-high, but with goblin-faces, sallow, leering, squint-eyed.... they reminded me at once of that Southerner at Bree: only he was not so obviously orc-like as most of these were."
That echoes the "squat and broad" and "tall and grim" distinction rather nicely - with the added detail that
some of the "tall and grim" people had orcish features. At this point, both Merry and Pippin and their audience (Aragorn, Gimli, Legolas, Gandalf) are more than familiar with the Uruk-hai of Isengard, but these man-high creatures were new to them (apart from the echo of Bill Ferny's friend).
Later in the conversation, when Merry says that "I don't think many orc-folk, of whatever size, escaped", that's clearly a reference back to the line about the "horrible" men with goblin-faces: that is, Merry doesn't think either the orc-sized or the man-sized orcish creatures escaped in any number.
Anyway, the key point here is that Isengard's soldiers seem to come in two main sizes: orc-sized and man-sized. At Helm's Deep, we learn that Gimli prefers fighting the former to the latter on size grounds, even at the point where "the hugest orcs" are mustered. And at the Isen, we hear that the orcs have a difficulty with the Rohirric shield-wall that the Dunlendings don't have. That's a nice echo of Gimli's comments. My inference is that Uruks are roughly the size of dwarves, with smaller orcs (like those Frodo and Sam join in Mordor) about the size of Hobbits. And at no point do we hear of Hobbit-sized soldiers fighting for Isengard.
I'd also argue that almost all of the orcs we see fighting for
Mordor are Uruks; Frodo and Sam's Durthang line seems to be a bunch of reserves being mustered in Sauron's final push; and we learn in the Appendices that Mordor has been issuing whole armies of Uruks for hundreds of years. Also, the small tracker-orc complains to his Uruk companion about the way "you fighters" have handled the war, which suggests - unsurprisingly - that it's the big soldier-orcs who do most of the fighting. And why wouldn't they, given that Sauron had whole armies of them to take Osgiliath and to send to the Misty Mountains (whence Saruman derived his Uruks).
So why do gamers almost aways include huge contingents of "lesser orcs" in Middle Earth army lists for Isengard and Mordor? If you look at almost any army list drawn up for Isengard or for the Pelennor Fields, there's always "Orcs" and "Uruks" as separate things. I don't think the text supports this at all - but because Tolkien was writing fiction rather than 'balanced' game factions, there's a problem. And this is it: the Uruk-hai appear quite formidable in close-up: fast, strong, fierce, heavily armed - heavy infantry who are also preternaturally fast and armed with bows. But in 'zoomed-out' scenes in the book, they die in their droves and are clearly inferior to Men.
That apparent contradiction is because Tolkien's writing a heroic narrative - and his heroes need to face daunting odds. And there's a trope in medieval heroic narratives in which "everyone's a better fighter than everyone else" - i.e. all combatants tend to be described as formidable fighters. Tolkien tends to adopt this, I think, which makes adapting his stuff for gaming slightly harder than initially meets the eye.
Anyway, a straight reading of LotR indicates that the orcish armies of Isengard were mainly or entirely Uruk-hai, fighting alongside Dunlendings and half-orcs. The Isen narrative should be read in this light, I think.
One more thing: people often say that that the wolfriders can't be Uruks. I've never understood this. Merry talks about "Orcs", some of whom were "mounted on great wolves". He doesn't indicate that these were small Orcs. And wargs seem to be able to carry large orcs - Beorn "pulled down" Bolg before crushing him, which strongly suggests that he was mounted. We can assume that he, like his father, was a "great orc", i.e. one of the Uruks that Sauron had sent to the Misty Mountains (hence the "black Uruks of Mordor" in the Mines of Moria.