I have not personally attempted something similar with mysql but. If I were going to try this, I would think about putting a load balancer server between mysql instances and the droplets that will connect to them. Perhaps something like HA proxy. We configure this proxy by telling it what its backends are and when to connect to each(perhaps a round robin) And then all your droplets would connect to the proxy server without caring how many sql you have.

When you want to add an additional mysql, it's a simple matter of settign up the replication, and adding its ip to the proxy list of backends so that it includes it as a possible reference...