I don't mean "if Gondor conquers Rohan, Rohan troops should be available to Gondor" No, of course not! If that ever happens, then not Rohan troops, but some troops that look like Rohan units without banners and symbols of Rohan (because those symbols are enemy to Gondor, how can they be recruited by Gondor?). For that case, additional Horsemen can be created with some different banner or symbols, and they can be recruited. This is not so necessary in my opinion, if such thing asked or desired (When Mordor player asks you to recruit Gondor soldier, after subjugation for instance), then it would need additional units (without white tree in Gondor's case, maybe it would be burned tree in this case
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I mean, if Rohan loses a city to Mordor or Isengard, when Gondor captures it, Rohan population gives units for Gondor's recruitment as colloboration in that suggestion. Same applies to any Good-Evil factions between themselves.
I gave answer to you this on Troll topic I think. Saruman is a Maia and knowledgable enough to attract orcs and wargs, by his own dark arts or just persuasion, it is not given explicitly in books if I remember right, left ambiguous. Sauron asks to Saruman to assemble the army via Palantir, but Sauron doesn't assemble that army by himself but via Saruman's persuasion of ork troops either by dark arts or just speech. Saruman thought to be rival power to Sauron too if he could eradicate Rohan in Helms Deep, he failed. Trolls are not seen among the troops of Saruman yet, unless Saruman increases in power even further; thus it is possible for Saruman to recruit them as well in my opinion on this base.
I don't like the idea of borrowing units. If you think you can handle the script work, no worries for me, but I believe that is increasing the memory usage even further with additional lines of coding in the memory after each turn. If I am wrong, please correct my words. Borrowing units make things complicated. But mercenaries are also a type of aid, if their upkeep is too high, player would disband them next turn, so borrowing can work that way too.
After reading your comment about Mumakil, I think their recruitment should be limited to their own habitat. For example Mumakils should be available only in desert or Harad habitat. Let's assume if Misty Mountains Orcs conquered Gondor, from Harad border only they can recruit Mumakils. If Harad extends to snowy Misty Mountains, Misty Mountains shouldn't be able to recruit Mumakils there. Because they are exotic animals which their living would be limited to their own habitat. Even so, higher upkeep than usual should be necessary, if Misty Mountains Orcs plan to drive them in snowy areas after recruiting close to Harad (higher upkeep: using additional medicine to increase immunity system of Mumakils, thick blankets to sleep in snow etc.
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For faction mercenaries of human and orcs, upkeep of most units should stay same. For instance if Rohan army pays Rohan Riders 500 gold, Gondor army should pay 500 gold too when they have chance to recruit. But for Mumakils, or some special units which their habitat or culture provides them cheaper rations or equipment, the upkeep can be higher for those units because their supply needs would be higher by logistics too. This applies to Elven and Dwarven units especially, let's say the Bow string of an Elven Archer is broken. That Elven archer won't be liking to use Human Bow Strings, but prefer Elven ones for his Elven Bow. Supplying such needs would increase the cost of employer faction. 15% additional upkeep would be fair I think (for transportation).