I love this idea, for it\'s a really common thing in most non-game universes, and only lacks rules for it\'s use ingame.
I\'m not going to dive too deeply into combinations and other rather obscure things, but there obviously need to be some restrictions. However, most of the ones I envision haven\'t been brought up yet, so here they are:
The ring can be made up of any number of mages and any combination, any races, any alignment and any experience level, but:
1) Each member of the ring increases the difficulty to co-ordinate the powers, just like large groups of ppl. are less easy to co-ordinate than smaller ones
2) The experience levels of the mages involved have a great impact. This means that a circle of 100 highly experienced mages is more easily controlled than one of 100 newbie mages. Also, each mage will need to succeed in the controlling and the co-ordination, and dependant on the power required for the spell, the complexity of the ring (adding all modifiers together, like size, etc.), this may be very difficult for a newbie mage. Because every mage will need to be successful, the number of low level mages in the high power circle will greatly reduce it\'s chances of success. The difference in knowledge can also add to overall complexity.
3) The ways the participating mages are proficient in will influence the complexity. Having water and fire co-operate will be more difficult than only water, or white and dark, or whatever. This will also apply to the spell being cast, and even more so to the mage casting the spell / controlling the circle, because anyone who doesn\'t \"fit in\" right will be
harder to co-ordinate (the fire mage co-ordinating two fire and one water mage will have problems with the water mage, but not so much with the two fire mages, as they are more \"comprehensible\" ).
4) The alignment of the mages can add to complexity. While they might have a common goal, the level of trust is important for effectively joining forces, because everyone needs to give up certain safeguards in order to share / receive power. Therefore, if there is one \"evil\" mage in the circle of \"good\" mages, they are likely to instinctively raise barriers against each other due to lack of trust, reducing the chances of successful co-ordination.
Also, the power of a dark way mage might just \"feel\" bad / wrong to the white way mage, or vice versa, even if it\'s just subjective and has no reason other than imagination.
5) Of course, a mage who is used to circle magic will be able to participate more easily, while one who may know much and be powerful but has never done this before will have difficulty participating and, at the same time, create co-ordination problems for the rest who need to compensate for the lack of experience and the added wildcards.
6) there should be absolutely no sexist restriction in it, because it doesn\'t have any place in there. After all, the WoT magic has been \"contaminated\", but the PS magic hasn\'t. Also, it\'s not good in terms of RL problems, which just shouldn\'t exist in the game world.
7) While it might, at first glance, seem reasonable to add problems with certain races, this is also a bad thing and will solve itself automatically due to the restrictions 1 - 5. A Kran will not reach the same level as a Human, therefore it\'ll be harder to integrate them anyway, no \"racism\" required.