My ASA have a default Global Service policy where it does Inspection.
And i wish to know is that if i apply an Interface Service policy which does MSS Exceed Allow for only HTTP/HTTPS/SMTP.
Is the ASA still doing the default Inspection as it's stated that it will override the default policy?
Yes, that should work.
Alternatively, you might want to turn it on for the whole box:
set connection advanced-options mss-map
service-policy global_policy global
Feel free to ping me @ work on sametime if you have more questions.