You can ommit the usage of the field / subject if you mention it before the current sentence or if it’s obviously the subject of your sentence.

If you just wrote the field, the usage of “en matière de …”, “sur …”, or “là-dessus” after “rayon” will be redundant.

Eux, qui en connaissent un rayon, y sont pour qqch.

If the context is clear enough from what was previously mentioned, the word “en” refers to [the French language in this case] and there is no need to specify more, unless there’s a desire to add emphasis.

The possibility to omit has very little to do with the expression being common, since in fact it is not so often used (at least on my end), but it is just common enough to be clear without more details if the general context provides them.

The same would apply for “s’y connaître”, “y” playing in this expression the same role as “en” in the first one:

Eux, qui s’y connaissent, ne sont point étrangers à ma rapide progression, loin s’en faut.

On the other hand, “être calé” is missing any indication about what we might have discussed earlier, so the complete absence of anything to apply it to is not ideal¹. However, one could add some very general term representing the specific subject being discussed without having to fear being too blurry:

Eux, qui sont calés en la matière (i.e. in the subtleties of the French language), y sont pour qqch.

_{¹ In a more relaxed language, “être calé” could perhaps be used as-is, but to the price of a potential ambiguity:}

_{° Eux, qui sont calés, y sont pour qqch.}

_{Now, one could wonder if They are experts in the French language (or whatever is being discussed at the moment), or if they are simply knowledgeable in just about everything about life, the universe and the rest, a polymath in the vein of Leonardo da Vinci or Gottfried Wilhelm Leibniz.}