Did any of the seven ecumenical councils respected both by Catholic and Orthodox Christians pronounce against slavery or allow slavery?
No, none of the ecumenical councils prohibit slavery, or deal with the subject to any significant extent.
The closest connection to slavery that I have found in the ecumenical councils is the Council of Chalcedon (fourth ecumenical council, AD 451), which ratified as ecumenical the canons of the Synod of Gangra (340). This local council condemned several Manichaean practices, one of which was encouraging slaves to flee their masters. The third canon reads:
If any one shall teach a slave, under pretext of piety, to despise his master and to run away from his service, and not to serve his own master with good-will and all honour, let him be anathema.
Thus we don't find any specific opposition to slavery here.
Another local council, several centuries later, did speak against one form of slavery – ownership of Christians by Jews. This council, the Twelfth of Toledo (681), placed a number of other restrictions on Jews as well. But since it was never ratified by a subsequent ecumenical council, it's only tangentially relevant to your question.
Of course, there are many church fathers during this period who opposed the practice, often vehemently, but nonetheless their views were apparently not widespread enough to be included in any of the ecumenical councils. Indeed, Wikipedia notes that even as late as the 15th and 16th centuries, several popes continued to call for slavery of non-Christians.