Now - yes lag is important to avoid, I get it quite a lot unless I use med. to low settings, so I really worry about it making stuff unplayable (such as this insanely and impossibly awesome mod). It would also be unrealistic and very limiting to have archers kill one unit at a time. On the other hand, one of my pet hates about BFME1 was not only that an entire horde of Uruks could climb a ladder at once (it was slightly more realistic with the Mordor siege towers), but that you could send multiple hordes up at once, effectively dumping your entire army onto the walls without even having to bring up any other siege equipment. IMO the uruks climbing ladders should be limited to a "really slow trickle" pace.
I believe I've thought of a compromise - How about you have hordes and individuals. The bulk of the army would be hordes. These would serve to be the uruks that recklessly throw themselves at the fortifications and get shot as a result. They would also form much of what attacks the gates. Then, of course, there are the individual uruks hanging around in the ranks, and are the only ones that climb the siege ladders (are also quite expendable when the Deeping Wall gets blown to kingdom come) and the giant ladders, if they end up being a feature.
On specifics, since you would want ladder-climbers present pretty much all of the time, there should be, say, 100 singular uruks on the map at any one time. When some of them are killed, more are spawned. Perhaps you could even spawn them at the base of the ladders so as to minimise travel time (and by extension get filled with arrow time). I do not think it would ruin the overall effect if they were spawning in an already crowded area.
Well, that was a longer post than I anticipated

Hope the mod turns out great, it's got huge potential.