Ticklestick ... the problem is you're using the term 'professional' much too easy. The only professional soldiers in the period were mercenaries, and as i said the few (mainly elite) guards and garrisons. Professional means: Directly paid, all the time of their service (just as modern soldiers ... but this pay wasn't possible in the slightest, thus an army was paid with the permission to plunder, pillage and loot - but normally not in the own land where a standing/professional army would be positioned, thus i said: campaigns).
All other (non-fulltime) soldiers were raised for campaigns and not used as standing armies, they were farmers and citizens or whatever as per culture, even if they got military training and experience as per culture and its military organisation, which usually was built on a militia system (despite the said exceptions ie. Carthage with very few militias). And, a militia soldier was not automatically a so-called levy, if you see 'levy' equal to inexperienced coward who runs away in a battle.
Now, same theme from mere sociocultural aspects: You didn't show a single source that brings evidence for professional
standing armies ... perhaps you don't understand what 'standing army' means? This is: Unlimited service in the army until the guy would die or gets his kind of pension due to age and time of service ... a member of a standing army is (and was) a fulltime-soldier. Despite the mercenary warrior and the few guards/garrisons, all other soldiers went home to their families after a campaign or were rewarded with a new home or simply took it from their enemies, and then worked for their usual life as farmers andwhatnot, until their leader would call them back to arms for a campaign, and they came if they were loyal enough (and usually it was traditional duty to serve as soldier, as militia soldier). Got it now, what is meant? Probably not. Just check out the societies of the periods, and you'll understand - these guys were peasants/farmers/fishers, some hunters, some other manual workers (buildings, metal etc.), few traders/merchants, few lesser nobles and still less rich nobles, and else many slaves, and then there were mercenaries (which were as well mainly complete normal guys who weren't able to afford a usual life and thus chose to be a mercenary/soldier, and few of them probably due to hunger for adventures) and few fulltime-soldiers. What do you think ie. Alexanders army was consisted of (from which the successor states generated)?
Back to topic: An according kind of soldier/recruitment pool is very fitting as well for the 'Rome' period. People who don't understand this just need some proper history lessons
