Normally you start at 1000 elo, but if you already have a 1v1 rating, then you start at that rating.
I think you already have a 1v1 rating based on you post, so you start at that rating for team games as well. For most people their team game rating is higher then the 1v1 rating. So feel free to join the TG ladder as well.
The reason why TG ratings are generelly higher then 1v1 ratings is because of a bug in the calculation. More about this issue in the following thread:
In general TG are less balanced then 1v1s in ranked. But TGs in ranked are still much more balanced then TGs in quick play.