Viscous force is the force of friction between the adjacent fluid layers. So, definitely there will be some value for coeffecient of viscosity for all fluids. According to the definition there is no fluid which is said to be ideal because all fluid can be compressed depending on its density, since the molecules are not that much closely packed when compared to solids. Also, less viscous fluid can be compressed more when compared to highly viscous fluid. So, according to the definition of Reynold's number, if a fluid is of zero viscosity it can be compressed as well and thereby the Reynold's number becomes equal to infinity. On the whole the definition set for an ideal fluid is right but there is no fluid which is ideal in this world.

Reynolds number (no apostrophe, by the way! :o ) is perhaps more clearly understood as a ratio of interial effect to viscous effect. Then the kinematic viscosity is perhaps the more appropriate parameter to consider, where the dynamic viscosity would only tend toward zero when the density also tends toward zero; therefore, the kinematic viscosity is always non-zero in the limit but of a to-be-determined numerical value in practice (in theory, who knows!). Furthermore, the parameter "d" in the inertial effect is arbitrary until a physical system is defined, where it may become a pipe diameter or a film thickness as appropriate, and the velocity is usually neither uniform nor constant through the region of interest but is often taken as some justifiably consistent average suited to the particular study. Finally, as a side note, there are different kinds of "ideal" fluids dependent on the behavior of interest. For the system of your "ideal fluid," the kinetic theory of gases might be better employed and Reynolds number becomes a different equation, as well, based on the conceptual definition mentioned in the first sentence, addressing the issue of the appropriate velocity term and perhaps even redefining kinematic viscosity on more fundamental/molecular terms rather than classic/continuum properties.