As a side-note, do we have to start as Chunin?
No. But it would help if your character is somewhat on the same wavelength as the other characters. Most will be around chuunin level (it is a very rough estimate), so going full Madara or retard-Naruto could be frowned upon. As per the canon, one can be a genin and still be on the level of chuunin. The rank of jounin is off-limits however as it could create a power-struggle in the village that we don't want.