Slavic Gaul : No. The population was amonst the greatest of Europe, even before the roman conquest. There's no way that another people could, except by migrating entierly (and it would even be a certainity of sucess) in Gaul.
Dacians : No, the gallo-roman elites were really well present and would manage to "infiltrate" any conquering minority as it did for Franks OTL. At best, you'll have a "Dacian" kingdom where Dacian means "elite" without real ethnic definition.
To quote Braudel, speaking about french population.
"Invasions that follow would lost themselves little by little in the population already present, submited, sometimes expelled out of their lands, but that will rise, expand, propser anew".
Population in southern Gaul in 100 AD : hard to say. Maybe 3, 4 millions.
Southern France : giving how much southern Gaul was hellenized and romanized even before the roman conquest, I'm pretty sure that it would be the LEAST credbily slavized/germanized land of Gaul in an ATL.
Barbaric level : again, as I told you on another thread, define "barbaric level".
If it's not knowing the pleasures of large-scale slavery-based economy, yes probably.
Without Rome exanding outside Italian peninsula, it's likely that Celts tribes and statelets will evolve in a situation analog to VI or even V century Greece. An invasion in Gaul at this time could be defeated (especially coming from a Dacian or Germanic people that would have certain logistic troubles, and if they kept tribal institutions)