The advanced classes (Summoner, Scholar, Paladin etc) do NOT start at 1. They automatically level with your main class. So a Summoner 35 is also an Arcanist 35. If you level either up to 36, the other is also 36 and so on.

However you start over at 1 for new base classes. So the class you level to 15 has to start at 1, but no you don't screw yourself over too much by doing the quests now. Just do the hunting log you get, as well as a few leve-quests (on the 2nd class you can get them at level 5 already or so), and that should easily get you to 15.