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My Structural Engineer suggested this:

[PB=Plinth Beam; SSM = Sizestone Masonry]

18" PB
3" DPC
15" SSM
---GROUND LEVEL----
18" SSM
6" PCC
----SOIL----

My Architect Changed it to

18" PB
12" SSM
----GROUND LEVEL---
18" SSM
6" PCC
----SOIL----

Questions
1. Is DPC optional ? Architect says it is, since RCC structure should avoid moisture issues (I still don't have a clear idea why DPC is needed)
2. If soil is hard (mine was rocky at most places), is the Architect recommended SSM ok ? Btw, is one area has rocky soil, is is true for all others areas (mine is almost 40*60)
3. Our PL (Plinth Level) is 2.5' looks like (assuming PL means top of PB). Architect claims 2' is the norm...comments ?

DPC helps in avoiding water going inside SSM work
Think like this, 3" DPC won't be too much of cost for you. Having it is only advantage. By not having it and later realize it is required will not be a good situation.
Also ensure top level of plinth is at leaset 1.5 to 2 ft high from road (middle section of the road)