Ace fights at the level of Shishibukai (blackbeard is stronger than the rest, but still), Aoikiji is probably the strongest admiral.
Admiral > Shishibukai.

as for Logia abilities... I guess that in close combat, their abilities would negate each others (ace's flame punch would hit, but only cause normal damage, but won't get the cold backlash), and in terms of element range attacks, defense against them would take far more energy than usual (and so would form re-constructment).