So, last game in middle of combat with a ghost a player of mine asked "can I speak to him?" and I answered sure, but I told him: since you're all in mid-combat you can use your turn to speak a small phrase and on the ghost's turn, he might answer you - or not. 

I'm currently allowing the PCs to talk only on their turn. No action required, they can speak freely, as long it's a short phrase. If someone wants to answer, needs to wait their turn as well

Is this accurate? how you guys handle conversations between PCs and NPCs in mid-combat?

As long as it's no longer than six seconds of dialogue, I'm fine with it and no action's required. Any more than that and I ask the player to rephrase, or have their sentence cut short until the next turn. I would be willing to have them expend their actions, bonus actions or reactions to continue if they deemed it a worthwhile sacrifice. I don't yet let vocal (V) components of spells interfere with dialogue of this nature.

Otherwise your system is fine. Things like dialogue in the middle of combat rarely break the game, and you'll be able to put a stop to it easy once you sense it might.

I allow conversation while in combat. Like Hex, I let a short amount of dialogue and would step in if I feel they are getting too close to a monologue conversation.

I tend to let players chat and deliver dialogue and such without sticking too strictly to the 6 second rule. I really only intervene if the talking is stuff that would directly influence the combat. Like... if two people are strategizing in-character, or if someone wants to meaningfully interact with an NPC. If a player wants to just sit and banter with the villain for a bit I usually play along, but if they try to persuade them of something I enforce shorter structure, and if they want to say more than a quick phrase they need to spend their action or bonus action to do so.

Quote from BKThomson >>

I allow conversation while in combat. Like Hex, I let a short amount of dialogue and would step in if I feel they are getting too close to a monologue conversation.

Quote from TransmorpherDDS >>

I tend to let players chat and deliver dialogue and such without sticking too strictly to the 6 second rule. I really only intervene if the talking is stuff that would directly influence the combat. Like... if two people are strategizing in-character, or if someone wants to meaningfully interact with an NPC. If a player wants to just sit and banter with the villain for a bit I usually play along, but if they try to persuade them of something I enforce shorter structure, and if they want to say more than a quick phrase they need to spend their action or bonus action to do so.

 Do you guys let them chat only on each one's turn? 

I think it's hard to quell table talk... some people will be in communication with the person next to them, maybe strategizing for what they intend to do next. I almost exclusively play online these days, and while that kind of forces players to be quiet when it's not their turn (since you can't really hear anything if everyone is talking at once), there's no real way to stop people sending private messages while they wait for their turn. So for the most part, what we end up with is whoever's turn it is generally talks more, and if they address another character I let the other character respond without forcing them to wait until their turn, unless it's something incredibly time sensitive. If someone says something and its not their turn I generally just let it slide, unless they're saying a lot and interrupting the flow of the game.

For short phrases, such as taunting a villain, I usually let the players roleplay it and assume it lasts over the course of the combat. If the players want to parley, I have them roll a CHA (persuasion or intimidation) check to stop the fight and enter a roleplaying scene with a foe, determining the DC based on the foe's nature and aggression, in addition to how the fight has gone so far.

Between PC's, I let complex strategization occur before combat most of the time. When they are ambushed, the more intelligent PCs can exchange coordinated ideas in short utterances. There's no clear rules on it, so I make judgement calls based on what sounds reasonable.

Quote from Licozo >>

Quote from BKThomson >>

I allow conversation while in combat. Like Hex, I let a short amount of dialogue and would step in if I feel they are getting too close to a monologue conversation.

 

Quote from TransmorpherDDS >>

I tend to let players chat and deliver dialogue and such without sticking too strictly to the 6 second rule. I really only intervene if the talking is stuff that would directly influence the combat. Like... if two people are strategizing in-character, or if someone wants to meaningfully interact with an NPC. If a player wants to just sit and banter with the villain for a bit I usually play along, but if they try to persuade them of something I enforce shorter structure, and if they want to say more than a quick phrase they need to spend their action or bonus action to do so.

 Do you guys let them chat only on each one's turn? 

 Casual banter back and forth I allow but it has to be started by/on the active player's turn.