Has there ever been any major interaction between the Eternals and Inhumans? Since they're cut from the same 'test tube people' cloth and all.
Yes there has been. Black Bolt encountered Ikaris. They fought at first but then became allies. The Eternals then helped The Inhumans to relocate all of Attilan to The Himalayas.
Thus occurred in a backstory in the pages of The Mighty Thor. Then aspects were retold in Jenkins and Lee’s Inhumans series.
Also, The half Inhuman known as Ransak has had many adventures alongside The Eternals.