Is it proper/ok to defines them as :
fromIntegral :: (a::Integral) -> (b::Num)
and
round :: (a::RealFrac) -> (b::Integral) ?

No; Integral, Num and RealFrac are type classes, not types. Think of a type class as a "set of types" which all support certain operations. For example, anything in the Num class must implement +, -, *, and a few other things. So a type signature like (Integral a) => a means "any type a, as long as it is in the type class Integral".

Is RealFrac is-a Num ?

Yes, any type which is in the RealFrac type class must also be in the Num type class.

Does the order matters in (Num b,Integral a) => a -> b or
(Integral a,Num b) => a -> b

No, the order of type class constraints doesn't matter.

With your encouragements, I'll keep pluuging. Thanks.

Good luck!

-Brent