E.g. if two main clauses are put together (with "und" for example) there is no change in word order whereas if it is a subordinate clause,the verb goes to the end. Is this a fair assumption? And if so, what about the verb and (nominative) nouns switching places?
That's right. In a subordinate clause with a conjunction, the verb goes to the end of the clause. As a result, the subject will (usually) appear right after the conjunction.
Ich glaube, dass der Zeuge lügt.
Even if we make the clause longer, the verb stays at the end:
Ich glaube, dass der Zeuge gestern während der Vernehmung auf dem Polizeihauptquartier gelogen hat.
However, there are cases where the subject does not immediately follow the conjunction:
Ich glaube, dass der Zeuge mich anlügt.
Ich glaube, dass mich der Zeuge anlügt.
These are both valid word orders. although the second is slightly less usual. But in both cases, the verb has to be at the end of the clause.
But this doesn't apply to all conjunctions; for example, after denn the verb stays at second position.
So, as a general rule, the position of the verb is fixed in a German clause, whereas the position of other parts of the sentence can deviate from the usual subject-object-time-place order, especially for the purpose of emphasis.