Minor changes in orthopraxis which don't touch faith and validity of the sacraments have always been allowed, especially when it increases the piety of the faithful and the intensity of the liturgy. Only the hierarchs can decide whether some "new" practice may be allowed or not.
There are enough proves that, when the sacraments are concerned, the form has been often changed dramatically, and the Church Fathers are a sufficient proof for that.
Faith is something different on this point. Only the ecumenical councils or a solid, continuous consent of the Church Fathers might allow a belief to be considered as official within the Orthodox Church. For example, the doctrines of the Trinity and the two natures of Jesus Christ have been formulated by the church in the Ecumenical Councils. Yet we consider prayers for the dead a well-established orthodox practice, since both the Church Fathers and the liturgical texts give a sufficient proof for it.
On the afterlife, as my brethrens repeatedly showed in vain to you RC's on this board, there are at least three or four alternative positions showed expressed by the Church Fathers on the afterlife.
1) There's heaven and hades. Heaven is a place of bliss, and hades a place of torment. This is the easiest reading of the Parable of Lazarus and Dives. Only tradition adds to this that some souls can be delivered by the prayers of the living.
2) There's a preliminary purification for everybody (apokatastasis) which automatically leads all to heaven. This doctrine of Origen seems to have been held partially or fully by st Basil and st Gregory of Nyssa, but the condemnations against Origen seem to have been naturally extended by theologians to apokatastasis too.
When, over long periods of time, evil has been removed and those now lying in sin have been restored to their original state, all creation will join in united thanksgiving, both those whose purification has involved punishment and those who never needed purification at all"
(Gregory of Nyssa, Catechetical Oration 26)
3) There's a preliminary purification for those going to paradise, akin to your purgatory. This is one of the "possible" interpretations, yet it seems that only st. Augustine and st. Gregory the Great subscribed to this belief at least in the first centuries.
"Each one will be presented to the Judge exactly as he was when he departed this life. Yet, there must be a cleansing fire before judgment, because of some minor faults that may remain to be purged away. Does not Christ, the Truth, say that if anyone blasphemes against the Holy Spirit he shall not be forgiven 'either in this world or in the world to come'(Mt. 12:32)? From this statement we learn that some sins can be forgiven in this world and some in the world to come. For, if forgiveness is refused for a particular sin, we conclude logically that it is granted for others. This must apply, as I said, to slight transgressions."
(Gregory the Great, Dialogues)
4) There's a particular judgment and expiation process named "toll-houses" before accessing either heaven or hades, where prayers for the dead are useful to deliver the souls of the dead from the power of devils. This is a very common belief of the great mystics of the East and was supported not only by st. John Damascene, Theophilus of Antioch and Athanasius; but also by some Greek and Slavonic liturgical texts.
"O Virgin, in the hour of death rescue me from the hands of the demons, and the judgment, and the accusations, and the frightful testing, and the bitter tollhouses and the fierce prince, and the eternal condemnation, O Theotokos."
(from the 8th canticle of the canon at Matins, by John of Damascus)
As you can see, purgatory is only one of the "possible" descriptions of the afterlife. Orthodoxy ordinarily supports positions 1) and 4), which aren't necessarily in contradiction but even complementary (since after the toll-houses both heaven and hades are identical to position #1). Some accept some other form of purification such as #3 and I don't think it's so dangerous but don't like it at all. Afterall, most of us Orthodox don't confess such a belief.
The RC definition of purgatory was a minority opinion among the Church Fathers and is now refused almost universally by all of the Orthodox and imposing it to the church would be a terrible error. The fact that the Council of Florence and Ferrara was never acted upon by the bishops (and the same can be said of the Panorthodox Synod of Jerusalem) is to me and many of us a sufficient proof that these are pseudocouncils and that the doctrines exposed therein should be analysed with extreme caution.
In Christ, Alex