To server small no of users, EPC Lite product can be used. EPC lite has all the necessary component of LTE network to provide LTE network services to its users. For emergency/defense services, network in a box is popular solution. Putting MME, SGW and PGW into a single box, does not mean all the interfaces such as S11, S1-U, S5/S8 have been merged. From the hardware point of view, all the nodes are packed together so that network can be deployed in fast manner.
Now coming to your question combining the SGW and PGW means, need to re-write application so that it can serve all the interfaces such as S11 towards MME and SGi towards external network. Traffic for S5/S8 interface would be mapped at the application level. It saves hardware cost and avoid data plane delay. Disadvantage what I realized is as part of mobility, # of handover messages will landed up more at the same combined node.