They will be able to go MIA to EIS without issues. EIS to MIA may be another story. I wrote a post a few years ago about what goes into determining max takeoff weight. There are a lot of factors and I don't think the can get out of EIS at max structural weight. They will be performance limited. There will be other issues such as dry or wet runway, winds enroute, weight of overwater gear, and the weather in MIA. They could block seats or hop to SJU for fuel. I also don't know what type of fuel reserves will be required into EIS since the airport not have any instrument approaches.
I will be very surprised if the FAA grants them a operating certificate by Jan. There is a lot of work that goes into that and they can't copy other airlines since the route is unique. If they start to show decent revenue however there is a new airframe that could fly the route. The CS100 would make a great aircraft on that route. They don't come cheap however!
G