I have some stuff to add to what you recently posted regarding the Republican and Marian units.
Given the start date on the eve of the 2nd Punic War:
1. It is highly likely that the gladius was already in use at this time as the main weapon of the Roman infantry. Polybius contradicts himself on this, but archaeological finds indicate that it was in use by the Romans in the First Punic War.
2. On the Socii Extraordinarii Pedites-You classify them as 'scouts,' but I don't believe this is an apt description. Polybius is basically the only source we have on their existence period, but he describes a picked body of infantry that were posted where the Romans were most vulnerable to attack. If they expected or thought a rear attack was a risk, they'd post them in the rear. When on the march, they were typically described as being in the front. When in camp, they were closer to the walls. I believe these were heavy infantry. Possibly the Socii version of the Triarii. It's possible the Romans simply picked the most fit of the allied soldiers. I would imagine it could have been done with political considerations in mind, as well. But I don't get the 'scout' description. Perhaps they were versatile and used in different capacities, but their positioning leads me to believe they were heavy infantry.
3. On the Socii cavalry - there is a designation of 'light' and 'heavy' cavalry, but based on the Second Punic War, the Italians at this point don't seem to have employed light cavalry at all. Hence the success of the Numidians and the importance placed on their inclusion in the army as time went on. Polybius describes early Italian cavalry as being lightly armored, but having at some point (probably from Pyrrhus or perhaps interactions with the Greek colonies) they adopted more Greek style armor, equipment, and tactics. Again going back to archaeological finds, they probably were heavy infantry with Greek spears, buckler shields, and armor (scale, mail, and plate are all depicted at various times). The equites could afford to buy themselves better equipment.
Light cavalry would have been a tactical innovation during the Second Punic War, I believe. The Romans did have Gallic allies who fought with them as well as Numidians and other allies who began to fill these niche roles.
4. One other thing to add to the Marian reforms - increased movement points for the units. The logistical changes of the time period were probably the single biggest change while the others seem to have been the end result of a gradual professionalization of the army.
Nice, job, I am particularly interested when, how and with what the velites and equites where replaced
I thought I could provide some info here, though the rosters reflect some of the issues. On cavalry - the wealthier classes had basically invested themselves into their landed estates. I would think there was simply less interest in serving in the cavalry and they had probably only a small body of actual Romans. With the granting of citizenship rights to all the Italians, there was less desire for the higher classes that constituted the cavalry to serve. Italian allies had made up the bulk of the Roman cavalry in the first place.
Auxilla mainly provided the bulk of the cavalry going forward.
In terms of light troops, with the poorer classes having the land requirements waived to join the legions, light troops, too, came from auxilla. Mercenaries were already employed in this role in the 2nd century.
I also have some issues with the Antesignani unit. I'm aware it's been included in the past, but the term is vague, had different meanings, and it's never clear in any source I've seen that it was in fact a distinct unit type. It seems to have typically denoted the frontline soldiers. Ceasar seems to use the term in this way as he describes taking these young men and giving them lighter equipment so they could fight with his outmanned cavalry forces. That would suggest it was atypical, and that they were typically more heavily armored. He also states he did this to give them fighting experience which again suggests simply younger members of the legions.
This was also another way Caesar and later Romans began to address their lack of cavalry (intermixing with light infantry). Caesar seems to have adopted it after seeing the effectivess of his German troops.
The term had different meanings at different times, but I really question whether it was a separate entity equipped in a lighter fashion. I think the term simply kept a relatively similar meaning to as when used by Livy to refer to members of the maniples who fought in the front ranks at any given moment. So, again, probably the younger soldiers. [though not always the youngest it seems, hard to say]
I would actually like to see a real representation of the mixed cavalry/light infantry units started (at least for the Romans) by Caesar. They would actually use the pilum as their main weapon and fight with the cavalry.