... from my understanding, there was an agreement and/or protocol between the Syrian Orthodox Church, the Antiochian Orthodox Church and the Melkite Catholic Church [of the East] to have an inter-Holy Communion and inter-Marital relationship.
These Churches also have an agreement to serve on one anothers altars. Please correct me if I am wrong.
There exist informal pastoral agreements between the Antiochians and the Melkites in their countries of origin. There is also a formal pastoral agreement between the Syriac Orthodox and Syriac Catholics and informal agreements between both of those and the Antiochians and Melkites. The provisions of these relate to pastoral care of each other's faithful in places and at times when their own presbyters are not available to them. They certainly all include provisions as to intermarriage between faithful of the various Churches and there is, unquestionably, provision of the Mystery of the Eucharist as well as, one suspects, those of Penance and Holy Anointing. It is simply a reality of the circumstances in which they find themselves in their own countries where, even taken together, Christians are a minority.
No provisions exist, of which I am aware, that permit concelebration of the Divine Liturgy, presuming that to be what you mean by serving on one another's altars. There are, however, instances in which they share the use of a single temple. You can find a detailed discussion of one such circumstance here
Another question may arise: Therefore, if the Syrian Orthodox Church [Non-Chalcedonian/Oriental] has inter-Communal relations with these Chalcedonian Churches, where do the other Oriental Orthodox Churches stand in this relationship? Are they in Communion by default [I'm not asking this, but I figure that this may be a question in the minds of others]?
I would say no. Although the Oriental Orthodox Churches do act in concert in many matters, this is not necessarily one of those circumstances - not to say that there are not instances. The Ethiopian and Eritrean Tewahado Churches have generally tended to shy from much in the way of such agreements, although the latter is likely to agree with such to the extent that it's elder Coptic Sister Church does so. It seems though that I recently heard that the Ethiopians were engaging in some dialogue, for the first time in a long while. The Copts have some agreements in place with the EO in Alexandria; an informal agreement between the Coptic Orthodox and the Catholic Church has existed for several years, but is not particularly functional any longer from my understanding.
The Armenians have any number of informal agreements in place; they, like the Syriacs, have a history of being open to both providing and accepting offers of pastoral care in instances of necessity.
The Indian Churches are, generally, not strongly inclined towards such, even those which are historically closely tied to the Syriacs.
See also this recent thread
, particularly the posts by Aristokles and myself, which address some related aspects of this matter.