If I say to an individual, who belongs to a higher social caste, that I will not help him but only those who are socially outcast and downtrodden, will it be considered as an abuse to the higher caste?
You need more context.
If you are giving a gift, it is completely fair to grant it to someone who is more in need.
But ethical action isn't always best modelled on gift giving. Should you "help" a higher caste person who had collapsed in the street? Of course.
One thing I really dislike is when people treat ethics like a zero sum game, so that any injury to someone belonging to privileged group helps those in an under privileged group. Not least because it always seems the marginalised within the former who lose out (white, but disabled and working class).
My answer was accepted already, but I had something to add, about an ethics of care. I agree that we should "care" for those that have a claim (be that due to our moral duty or something else) to that. It seems natural to suppose that the underclass have more of a claim for our kindness, but that needn't end up in abuse of anyone else.
Almost certainly not. You are talking about positive discrimination, or affirmative action. Different jurisdictions have different frameworks for what is allowed. But the core principle is correcting for past discrimination, is not discrimination.
See the discussion here for a detailed answer: Paradox of resolving discrimination
“A society that has done something special against the Negro for hundreds of years must now do something special for the Negro.” - Martin Luther King