if I chose TBC timeline, but do quests in Cataclysm zones (Silverpine forest) for example, is there any downside for this? Do I get lower xp from quests?
Basically I want to que for TBC dungeons, while leveling in Vanilla zones.
Nah. The quests scale to your level so they give the same xp wherever. When I start a new character I usually open up legion/mop/wod right at the start before I pick one to level in. Makes no odds.
It’s worth saying that there are definitely faster and slower choices, but that is up to you. It’s only a difference of an hour or so either way.