What quest are you trying to do? The quests in the THM guild start from level 1 and increase every five levels. If you're trying to get a quest somewhere else however (such as a main story quest), it can be any class you can do it as, as long as that class is the right level (in this case, 50). That means you can either wait til you have THM levelled up to 50, or else you indeed would have to switch to PLD to undertake them.