What you are asking is possible. In fact, virtually every utility in the world does this exact thing.
If run isolated from a utility 'grid', one (best if the largest) of your generators needs to be able to govern the frequency of the isolated system to, say, 60Hz.
This is a very complex topic, and I strongly recommend the services of a competant engineer experienced in this area before you commit any real $.
Assuming all generators are compatible (or made to be compatible via transformers), the total output would be the sum of the rated outputs of the individual units at the actual frequency and voltage (less any losses).