Valens the Arian Emperor

Johannes Quasten, Patrology, vol. III: The Golden Age of Patristic Literature (Westminster: Christian Classics, 1990) 205, 237-238, 255.

In his fight against state-supported Arianism [Basil the Great] combined ceaseless activity with wisdom and prudence. (205)

In 379 the small Nicene minority at Constantinople turned to Gregory with an urgent plea to come to their aid and to reorganize their Church, which, oppressed by a succession of Arian emperors and archbishops, hoped for a better future now that Valens had died. (237-238)

[Gregory of Nyssa] has left us (Ep. 6) a vivid account of the triumphal reception which was given to him, when he returned to his diocese after the death of the Arian Emperor Valens in 378. (255)

I don't understand what Quasten means in the second quote about "a succession of Arian emperors," since the only Arian emperor of the Roman empire whom I could find (on wikipedia, I admit!) was Valens. Julian (r. 360-363) de-Christianized the empire, his successor Jovian (r. 363-364) re-Christianized it, Valens (r. 364-378) was an Arian, and Theodosius I (r. 379-395) made Nicene Christianity the official religion of the empire.

I found these quotes interesting because I didn't know that the Arians were ever in power. As far as I knew, the Council of Nicaea hashed out the whole Trinity issue in 325, and the Roman empire abided by its decision. Apparently, not everyone did!

What's strange is that wikipedia presents the Arian emperor Valens as co-ruling with others: Valentinian I, Gratian, and Valentinian II. Gratian and Valentinian II also ruled with Theodosius I, the Nicene. Did they go with the Arians when Valens ruled, only to switch to the Nicene position once Theodosius came into power? I mean, if they were committed, die-hard Nicenes, then would Valens have been able to get by with his Arian oppression of other Christians, which Quasten depicts?