What is the best Pro-fractional resurfacing settings for acne scars for an Indian with brown skin?

Since you are a darker skin type, providers have to be careful with your skin. Darker skin types are more prone to post inflammatory hyperpigmentation from intensive laser treatments. Acne scarring treatments may take several sessions for you to see the difference. 

You are correct that Profractional can go down to 1000 micron.   However, if you do go down that deep, there is more risk of having side effects, like hyperpigmentation.    
150 micron is a safe setting to see how your face reacts to laser treatment.   If no side effects, the second time your provider will go down deeper, like 300 microns.     You have to do this in a step wise fashion to avoid complications. 
If your provider went down to 1000 micron on his first treatment, and you had side effects or complications,  the damage would be much greater than if they did the first trial at 150 micron. 
So this provider is absolutely doing it right and correctly, wanting to give you good results, but not at the expense of having more risks and complications. 

Yes, to answer your question if one has only one tool to treat your scars, 400 to 600 microns, low density is fine. Pre treat the area with tyrosinase inhibitor. I personally would use an RF device, but I also use ProFraction in fully ablative modes, fractional mode- I use to perform, however better options now a days. Al the best, DR Davin S. Lim. Brisbane. Australia