I know it is highly recommended to use Java 8 for elasticsearch and logstash. Is it highly recommended on machines running just the logstash-forwarder also?
logstash-forwarder is written in Go and isn't dependent on any JVM.
Or are you conflating the logstash-forwarder program with Logstash proper acting as a forwarder? In the latter case you should be fine with Java 7.