Another vote for "Queens" being seen as a machine rather than a leader. In fact, all ants should see other ants as tools and nothing more. The whole [we have no leader] thing is, I think, cool.
But hive mind is stupid, the way most people mean it when they say that.
Antmen shouldn't be stupid. They shouldn't be stupider than dwarves or humans (well, in general) - it should be their motivations that are different. They don't have individual aspirations - they can't breed, they don't care about their own lives, they just have a job to do. They don't want to do it - they just do. They're intelligence is... focused, if you will. They are not creative, they are not really motivated to do things other than what they are supposed to do, but they should not be stupid.
Communication: Not really a need to define this. I assume it would be a combination of scents, touch, visual movement, and various sounds from clacking to stomping the ground, but its not important. I could see communication with antmen being possible - but it would be difficult. Other races would have to learn how to talk to them, rather than the antmen learning to talk to other races, which would be difficult mechanically. As a bonus, Antmen do not understand deception. IF you can get them to listen to you, it would probably be fairly easy to convince the hive to ally with you.
"Art": Antmen hives are lined with weird looking sculptures and engravings. But don't be confused - antmen don't do it for the aesthetics. This is communication, they're writing system, and every line conveys information. This is something no other race is ever likely to decipher, unfortunately, because its super dense and super complex information. Some, at least, appears to be "maps" that contain the information an ant needs to travel from one part of the hive to another it has not been to, or even to other far away hives.
Special Castes: I don't think the ants even need an overclass, a group of planners or "intelligent" ants. They just understand, individually, how to do things, and they naturally agree assuming an idea is communicated by another member of the colony (which makes them very weak to any creature that could pass itself off as a member of the colony and start spreading disinformation). The only special "thinking" caste, would be the class that handles diplomacy with other hives. They're intelligence and history has allowed them to realize that avoiding mutually destructive conflict allows better survivability - thus arose a small class whose sole purpose is to defuse potential conflict with other hives. These "diplomats" are as close to "outcasts" and "deviants" as ant society contains, since so much of their brain is wired not to best understand their own role, but to understand and consider the roles of others. Other ants generally deal with them as little as possible - it is found they can have extensive detrimental effects on the colonies cohesion if allowed to mingle unchecked, since they often replace old ideas with new ones. They, of all the ants, have the most potential for creativity, empathy, and innovation. Which isn't saying much. If any ant ever were to decipher and figure out the language of another race, it would be a member of this caste, and they'd never be good at it.