"What kind of skills should the allies have?" - This part confuses me, so I'll try to answer the best I can.
Tactic skills are skills used to better buff Mixed Emblem teams. Because Mixed Emblem teams don't have "as much" in terms of easy access to +6 buffs, while other "Emblem" teams do (Hone/Fortify Fliers, Armor, Cavalry, Dragon (even though "dragon" isn't a movement type...)).
Tactic skills are best used when you have units with different movement types on your team, two being the maximum to take full advantage of the Tactic skill. For example, if you have 1 Infantry, 1 Flier, and 2 Cavalry units on a Tactic team, then all 4 units will receive whatever "Tactic" buff the team has.
On the other hand, if you have 1 Flier, and 3 Infantry, then only the Flier will receive the Tactic buff (provided one of the Infantry units have said Tactic skill since the unit holding the skill doesn't receive the buff).