Elite forces/units in the Pre-Marian army (early- middle republic)

[Timus]

I was wondering reading this thread, what would be the earliest reference to an ad-hoc picked force of legionaries ? Something along the lines of what "Caesar" describes a few times in the de Bello Civili.

By Caesar, do you mean him favoring the 10th and using them as his "bodyguard"? Or do you mean Crastinus' picked men (uncertain what type of unit that was)? Or Caesar's favoring of cohorts of evocatii?
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I was thinking more like those examples here :

BC 1:57
Brutus was far inferior in number of ships, but Caesar had assigned to his fleet the bravest men, front-line men and centurions, picked from all the legions, who had demanded this charge for themselves.

 Erat multo inferior numero navium Brutus; sed electos ex omnibus legionibus fortissimos viros, antesignanos, centuriones, Caesar ei classi attribuerat, qui sibi id muneris depoposcerant.

Or BC 1:64
So he orders the weaker men, whose spirit or strength seemed unequal to the effort, to be set aside from all the centuries. These he leaves with one legion to guard the camp. The rest of the legions he leads out lightly equipped, and after placing a great number of pack-horses in the river above and below leads across his force.

Itaque infirmiores milites ex omnibus centuriis deligi iubet, quorum aut animus aut vires videbantur sustinere non posse. Hos cum legione una praesidio castris relinquit; reliquas legiones expeditas educit magnoque numero iumentorum in flumine supra atque infra constituto traducit exercitum.

Or BC 3:84
But in the case of his cavalry he retained his previous custom which we have explained above: since they were many times inferior in number, he gave orders that lightly equipped youths from among the first-rank men, with arms selected with a view to fleetness, should go into battle among the cavalry, so that by daily practice they might win experience in this kind of fighting also. The result of these measures was that one thousand horsemen, even in the more open ground, ventured, with the experience they had gained, to sustain the attack of seven thousand Pompeian horse, and were not greatly terrified by their multitude.

Superius tamen institutum in equitibus, quod demonstravimus, servabat, ut, quoniam numero multis partibus esset inferior, adulescentes atque expeditos ex antesignanis electis ad pernicitatem armis inter equites proeliari iuberet, qui cotidiana consuetudine usum quoque eius generis proeliorum perciperent.

And I guess you could indeed add that one to the list though it's unclear whether those men where evocati like Crastinus or simply volunteers currently serving in the army and possibly in Crastinus former cohort (The 1st) 
BC 3:91
Having said this, he ran forward first from the right wing, and about one hundred and twenty picked men of the same cohort, serving as volunteers, followed him.

Haec cum dixisset, primus ex dextro cornu procucurrit, atque eum electi milites circiter CXX voluntarii eiusdem cohortis sunt prosecuti.

I have a vague recollection of reading that Catilina used "picked men" to fill its first rank but I can't remember the source. I wouldn't be surprised if there were similar references regarding Marius' or Sulla's armies and I'm wondering what would be the earliest one and if any dates back to before the late republic.