I have the Agema velites and legions sets (assembled and painted), and a box of the Victrix Allied legions figs (not yet assembled).
Both are excellent in their own right, but IMO they do not fit well with each other because the styles are different. here's how they diverge:
Agema have "true 28mm" proportions, so the figures are relatively slim. Victrix are more conventional stylized (relatively bulky) 28mm figures.
The difference shows up in 2 ways that you might notice when they are on the games table: 1) the Victrix heads/faces are larger than the Agema; 2) the Victrix arms/hands are bigger.
The shields are also a bit different betwen the sets – the Agema shields are actually somewhat taller and wider.
I found that Relic Miniatures republican roman metal figs are actually quite compatible with the Agema figs (I bulked out my Velites and Principes with them), although the Relic scutum shields are a bit smaller, so I gave mine spare Agema shields.
I've never seen the AB figs, so can't help with that. Hopefully someone will chime in.