Ok, I agree allowing for complete exchange of troops for the 35% requirement would end up with people bringing only 1 type of thing. But I still think it makes sense to allow for some benefit in this manner for guild members. In warfare you DO bring your best, and allow an ally's strength to support your weakness.
So I think you actually had a great idea of the 10% from others allowance, it allows coordination/teamwork, but doesn't remove the need for other units in individual armies.
As far as setting it up, make it part of the lobby joining. I will attempt to explain.
Have 2 or 3 guildmembers agree to coop BEFORE they submit their troops. This will send them into a group army page. There will be 2 info windows shown;
1) which stands as normal, with the personal contribution of each player's units in AP and % of army. This will flag red if the player doesn't have the proper 25/75% ratio, since that is mandatory for every individual.
2)A total submitted weight value, which looks at the overall unit strength of all players involved. This total would flag red if imbalanced from the 35/65% ratio. It would also allow the team to discuss rebalancing units. "Hey, we barely have any artillery, can someone toss in one of those?.... what about a few more archers since we're infantry heavy" Granted, in order for this to work there would obviously have to be some form of chat available. (I suspect an extension of the coding from the actual field combat into this room would be easy enough to code. You've already got those players separated into their own chat area, just make them enter it once they join the coop coordination room)
I don't want to move the requirement for lights/archers. If that happens I'd REALLY feel like i wasted all that time leveling them. I just want to be able to exercise my ability to, and thereby feel a benefit from, my researched higher-tier units. That doesn't happen if I need 9 lights/archers to have a 1 heavy, 1 cav, and 1 arty on the field.
Right now my army consists of 2 heavies, 2 lights, 2 archers. The heavies have poor gear/unequipped heroes, and one of my lights has a hero costing 65 AP (solely so high so as to allow the proper AP balance)