One thing you have to keep in mind is that technologies aren't 'set pieces' just waiting to be invented. The Lorica Segmentata is just another application of a known concept: plate armor, with the only exception that it was produced on a grand scale. Similarly with other innovations, they are a result of accumulated improvements over time.

Secondly, the Romans had a habit of absorbing and adopting successful tools from their enemies. The gladius, for example, is believed to be adopted from the Celts.

So, to answer your question, no, they didn't hire a brilliant inventor, nor was there a brilliant inventor simply submitting new inventions. Typically, things simply changed over time with small improvements over a period.

I don't know the answer specifically but equipment was not standardized until the military became a profession and not a duty. By the time that happened the empire was already large enough to have a wide variety of cultures and technologies to draw from. Before that time the legions were self-equipped so there probably was a wide variety of styles.

Semi-related follow up:

When and where did the concept of "intellectual property" come about? And inventing as a trade?