Which hero is best suited for the Conflux?

yes, +2 attack for Lacus is a big help, and +1 speed for the Water Elementals will be useful, even if they don't quite reach their full potential.

and there's also +5 advanced tactics, and on the next level, +7 might drop (expert tactics)... of course, if you want to play with Water Elementals... but in general, this will also help the Psionicists with their area-of-effect attacks, especially since you can't speed them up.