Google's improved quick reply notifications in Android 7.0 can make replying to messages easier, but it's still tedious to manage multiple simultaneous chats. That's doubly true of they're taking place in different apps. Flychat aims to make it easier by bundling all (or at least most) of your messaging apps into floating bubbles. They're chat heads, basically.
Setup is quick—all you have to do is allow the app to access your notifications. Although, if you want to avoid duplicate on-screen nags for each message, you might consider disabling peeking on apps you intend to use with Flychat. Similarly to Facebook Messenger, each apps gets its own bubble that appears on the screen when there's a new message. However, these can be from a number of apps, all of which are active at the same time. In Facebook, it's one bubble per contact.
Right now there's support for WhatsApp, Telegram, Hangouts, Line, Skype, Twitter, Threema, Textra, Facebook Messenger, and Plus Messenger. Each of these can be turned on and off individually. The basic app is free with some ads in the settings, but you can upgrade to the full version for $0.99. That gets rid of the ads, and includes resizable bubbles and custom colors.