Understanding the essence of Scala and the nuances that define its pros and cons is a prerequisite for organizations on the lookout for top-tier Scala developers. Thriving in the realm of concurrent programming, Scala, an acronym for "Scalable Language", is a statically-typed language that combines the best features of object-oriented and functional programming paradigms.

In the fast-paced technological ecosystem, where agility and scalability are the driving forces for success, Scala's appeal is hard to ignore. It's akin to the Fibonacci sequence in mathematics - simple, elegant, and inherently powerful. However, hiring a competent Scala developer is akin to finding a Fibonacci number in a sequence of prime numbers; it requires precision, understanding, and a fair bit of strategic planning.

To begin with, it's crucial to comprehend the functional and object-oriented elements of Scala. Unlike traditional object-oriented languages, Scala’s functional elements make it easier to manage and control side-effects, leading to a more predictable and maintainable code. This dual nature means a top-notch Scala developer should not only be adept at understanding and writing Scala code but also have a firm grasp on the fundamental principles of functional and object-oriented programming.

Furthermore, it's crucial to assess the prospective candidate's understanding of concurrency. Concurrent programming is the simultaneous execution of multiple task sequences, a feature that Scala handles with aplomb. Through its actor model, Scala, as compared to traditional multithreaded models, provides a higher level of abstraction for managing concurrency and parallelism. It results in robust and fault-tolerant systems, hence an understanding of the actor model and concurrency is a must-have quality in a top-notch Scala developer.

But, how does one measure the proficiency of a candidate in Scala? A crucial consideration would be their ability to work with the Scala Build Tool (SBT). SBT is a build tool specifically designed for Scala and Java projects, akin to Java’s Maven and Ant. It provides a testing interface, continuous compilation, and code coverage tools, making the software development process much more efficient. A Scala developer well-versed with SBT is undeniably an asset.

Moreover, experience with the Akka toolkit and Play Framework, both of which are written in Scala, are indicative of a candidate's advanced Scala skills. Akka enables the creation of high performing systems, while the Play Framework is a high-velocity web framework for Java and Scala. A developer familiar with these tools will certainly be comfortable with Scala’s design patterns and ecosystem.

A factor to consider while hiring a Scala developer is their understanding of the language's interoperability with Java, which makes it possible to call Java methods, access Java fields, inherit from Java classes, and implement Java interfaces in Scala. This makes Scala an attractive proposition for organizations with a significant investment in Java as it allows for a smoother transition.

Furthermore, familiarity with libraries such as ScalaZ, Cats, and Shapeless, which provide advanced functional programming tools, can be a useful yardstick of a developer's Scala prowess.

Yet, as Max Planck, the renowned physicist, expressed, "Science cannot solve the ultimate mystery of nature because we ourselves are part of the mystery we are trying to solve." Similarly, while technical skills are paramount, the human aspects matter too. Hence, factors such as communication skills, ability to work in a team, and a knack for problem-solving should not be overlooked.

Finally, the process of hiring should not be confined to the present. Scalability, the essence of Scala, should be the guiding principle when hiring a Scala developer. Hence, it is crucial to consider their potential for growth, adaptability, and their enthusiasm to evolve with the ever-changing technology landscape.

In conclusion, the task of hiring a top-notch Scala developer is a delicate dance between understanding the technical intricacies of the language and assessing a candidate's potential to take on the challenges of the future. With the right mix of technical prowess, professionalism, and passion for learning, you can indeed bring onboard a Scala developer who will be an indispensable asset to your team.