"касательно" is not normal preposition, it's impossible "поговорить касательно субсидий"
Usually it's used as "вводное слово" (parenthesis?) in combination with Genitive:
"Касательно моего желания или нежелания работать по выходным, ..."
and in this regard behaves alike "насчёт", but unlike the latter, should be avoided in the colloquial speech.

But we can always поговорить про ПРО

And don't forget that "о" demands Prepositional, whereas "про" - Accusative.