With a NEET UG AIR of 15,500, Maharashtra State Merit List (SML) rank of 1,200, and an open category state rank of 550, your chances in the second round of Maharashtra MBBS counseling for Mumbai colleges are realistic, given the reshuffling and vacancy movement that typically occurs after the first round. Many candidates with similar ranks secure seats as higher-ranked students vacate seats for other preferences or upgraded choices, leading to closing ranks dropping in popular colleges.

Here are some well-regarded Mumbai medical colleges where your rank typically falls within or close to the expected cutoff range for the second round:

1. Grant Medical College & Sir JJ Group of Hospitals, Mumbai
   One of the most prestigious medical colleges in Maharashtra. While the first-round cutoff might be slightly higher, second round offers have historically included students in your merit range due to seat shuffling.

2. Seth GS Medical College & KEM Hospital, Mumbai
   Known for robust academics and clinical exposure, this college fills up quickly but sees moderate cutoff movement in subsequent rounds.

3. Lokmanya Tilak Municipal Medical College (LTMMC), Sion, Mumbai
   Highly reputed for teaching and research, Sion Medical College nearly always has a few seats available in the second round, attracting candidates in top 700–1,300 state ranks.

4. Topiwala National Medical College (TNMC), Mumbai
   TNMC’s cutoff is sometimes marginally lower than the top three, making it a strong contender for round two admissions for candidates with ranks near yours.

5. Cooper Medical College, Mumbai
   Cooper is emerging in popularity and academic facilities; its cutoffs are slightly more relaxed, increasing your odds of allotment in second or mop-up rounds.

In the second round, closing ranks for these colleges often shift favorably due to reallocation and vacant seats. It's advisable to fill all possible options—including newer government and private colleges—through the counseling portal for the best chance at securing a seat.

Stay proactive by monitoring official counseling updates, cutoff notices, and filling a broad range of choices to maximize allotment probabilities.