i'm going to go against the grain here and say i really approve of the global mana pool, especially if it regenerates non linearly. i don't think there's any need for individual mana pools. with magic as powerful as it is (and there's nothing wrong with the power levels) making channellers autonomous units in any sense just makes balancing far too difficult. in my last game i got 7 channelers with only one imbue (my spouse, spawning a daughter and two sons, each gaining me a daughter in law) and won the whole game without building any troops, just summons (and i almost didn't even need them). that's obscene.
channelers were never supposed to be the source of magic in this game. magic comes from the shards. the value of channelers should be logistical (being in more places in once) not in overall power. this is the only way they will ever be balanced in a strategy game. the cost of creating channelers should be a sacrifice of individual power and reducing stats. use one stat to limit the level of spells the individual can cast and another the power of spells that are cast by that individual (currently int). one or both of these is then reduced by imbuing (and, i believe, having children, though to a lesser degree).
it works best to my mind if you regenerate mana along a non linear formula like this
where x is turns and y is mana. the equation is mana = max mana x (1 - e ^ (- game constant x turn number))
this way mana regenerates at a decreasing rate until it reaches your max mana. the higher your max mana, the faster the generation. you increase the max mana with every shard you own, for some unique buildings. every summons or enchant reduces it slightly. this has a lot of advantages over linear regeneration (ie, +2 regen for shard, -1 for summon) in that it's impossible to give the player negative or static regen if they summon too much. it also favours people who actually use the stuff, rather than stockpiling mana (though that is still a good idea). finally it governs both the size and the pool and it's regen in a single stat.
then the player has only three variables to manage. (and the designers have only three variables to balance)
1 - how big is my mana pool and how quickly does it regen (see graph)
2 - how powerful spells can this channeler cast (essense, will effectively stop your hundreds of channelers blowing all the mana in one battle)
3 - how powerful are spells cast by this individual (int)
if anything, i'd increase the size of the pool (and thus the rate of regen) slightly for every channeler. the downside is that it takes you longer to get to cast the big spells and you get less bang for your mana because spells aren't being cast by a skilled master with high int. your strategic spells expecially take a big boost from doing this hoarding power. you choose between being sauron, or the emperor, accumulating personal power, or an army of heroes with human level power pumping out medium magic. you can't let people do both.
3 variables with checks and balances that represent everything the game needs to and are easily balanced for a strategy game.