The Melcene people were considered one of the 'godless' ones, meaning their people weren't selected by any particular god and thus remained atheist. However, they eventually merged with the Mallorean empire. During the merger, were there temples to Torak allowed on their land, or were they 'encouraged' to join the Angarak religion?
Yes, but they never cared much about worshipping him.
From the introduction to Demon Lord of Karanda:
The integration of Melcene and Angarak was turbulent. But in time, the Melcene patience won out over Angarak brutality. Unlike other peoples, the Melcenes were ruled by a bureaucracy. And in the end, that bureaucracy proved far more efficient than the Angarak military administration, By 4400, the ascendancy of the bureaucracy was complete. By that time, also, the title of Commander in Chief had been forgotten and the ruler of both peoples was simply the Emperor of Mallorea.
To the sophisticated Melcenes, the worship of Torak remained largely superficial. They accepted the forms out of expediency, but the Grolims were never able to command the abject submission to the Dragon God that had characterized the Angaraks.