The types of synods range from the smallest (diocesan council) to the largest (“General Council” or “Ecumenical Council”), and there are seven different types of synods in all. The Second Vatican Council, for example, falls under the category of an “Ecumenical” or “General” council, meaning all of the world’s bishops were gathered to discuss matters pertaining to the universal church.
Ecumenical councils, of which there have been 21 in the history of the church, are the only kind of council whose decrees “bind all Christians.” All others exist to foster discussion and provide guidance on a regional level, but their decrees aren’t seen as infallible or binding on the whole church.
The Synod (or Council) of Laodicea, held in the 4th Century, appears to have been the equivalent of a “provincial synod”, meaning basically that the bishops in the region surrounding Laodicea were gathered to provide guidance to the faithful under their care.
The church allows for enough freedom of preference, outside of doctrine and dogma, that bishops are able to run their dioceses, in large part, how they see fit. The same holds true for regions and provinces of bishops as well. Any contradictions or problems in faith, morals, or discipline that come to light from a lesser synod or council would be corrected by the highest level within the church — namely the pope or the offices within the church he directly oversees.
In this particular instance, the bishops in the Synod of Laodicea seemingly spoke most commonly on “judaizing”, more specifically, that Christians were forbidden from judaizing if they were to remain in communion with the church. So now we ask, what is judaizing?
Judaizers were a Christian sect in the early church who believed that Gentiles must convert to Judaism in order to accept Jesus Christ as the Messiah. Judaizing amounted to following the Old Testament customs of the Mosaic Law instead of the teachings of Christ, effectively walking backward in faith to follow Mosaic custom. Ignoring the fact that Christ’s death brought about a new covenant — literally a “New Testament” — and the fulfillment of the Jewish faith, thus making salvation a matter of faith instead of a matter of following Jewish law, would naturally be rejected by the church Christ founded.
These canons in particular were meant for a specific time period and place and thus weren’t binding on the whole church.
As for whether any particular conduct constitutes sin, or at least mortal sin, you would have to examine whether it is “grave matter”, of which you were aware when you did it, and whether you did it voluntarily.
Catechism section 1857 "For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”