You can flower whenever you want. Some people give no veg time at all and flower from seed. Some strains don't respond the best to 12/12 from seed but in GENERAL flower whenever you want.

The more the veg time, generally, the more you'll harvest. Space is the main issue for a lot of growers, thus impacting how long they veg it.

6 inches and 6 nodes sounds normal. If anything, it sounds very compact. Without pics, hard to tell. :stoned: