There is no preference as to whether subordinate clauses precede or follow the main clause. Considerations of emphasis and effective writing might come into play.
As for the placement of subordinators, they are generally placed at the beginning of the clause, but can be placed as the second element or even later, especially in poetry, but not at the end of the clause.
in Galliam pervenit postquam
-- This seems very strange, even for Latin poetry. "In Galliam postquam pervenit" doesn't seem at all unusual, but "pervenit postquam in Galliam" does seem strange.
Latin word order is very free, since the syntactic structure is encoded in inflectional endings, not in word order, as it is in English.