Can I use the ADM1278 monitor 12V / 200A?
What should I pay attention to when designing? (EX: Parallel MOS/Rsense)
Is there an Excel tool like ADM1270?
200A is a lot of current. As stated your question doesn't contain enough information for a complete answer, but additional details that will help include:
1) Steady-state current (200A?)
2) Current limits (circuit breaker limit, severe OC limit, etc.)
3) Load capacitance
4) Tolerable start-up time (for inrush)
5) Types of FETs that you are considering (ie IRF2804S, PSMN0R9-25YLC, etc)
6) Environmental temperature range
7) Environmental profile (card in a backplane? industrial environment? automotive? etc.)
The primary concern for this design will be FET SOA at the current limits. You can parallel a bunch of FETs (probably 6) to reach your DC current, but during current limiting (inrush and faults), count on only one FET carrying most or all of the load. The alternative that you really should consider is using one of the dual-gate topologies, such as the LTC4282. These can control different FETs in different ways to handle DC and stresses independently. This will make your job A LOT easier.
I do have a pre-production design spreadsheet that you can use. I will email it to you.