Well, you can run a load balancer or another simple server alongside other services (given no port conflicts) on an existing droplet. Configurations of a server are like opinions. Everyone has a slightly different one, some may be more acceptable than others. There are also bad and good opinions. The choice is yours.
For load balancers though, the theory behind it almost dictates you should put it on another virtual machine. Because if you have three web servers and 1 of them also have a load balancer on the virtual machine, if the load balancer gets bogged down due to overwhelming website traffic, your whole system is down. In the best case scenario, the virtual machine is free to do nothing but balancing to avoid being a point of failure due to traffic levels.