Anything, no matter what fuel it uses, that generates power onto the grid, has a reserve requirement. How much that is and whether the existing reserves are adequate to cover it depends on the size and location but mostly on the size. A new 10MW Diesel generating onto a 20,000MW system is surely covered by existing reserves. An new 1100MW nuker on the other hand, will increase the reserve requirement by the difference between the largest unit before the nuker came on line and the output of the nuker if it becomes the largest unit on the system. If there was already an 1100MW plant, then reserve requirements do not change because of the size of that unit. They may change for other reasons, but not for the size of the unit.
The flows on tie lines from neighboring systems may also require reserve depending on how high the transfers into the system are. Trips of lines that are carrying transfers out of the system to a neighboring system result in an overgeneration condition and so only require the backing down of generation to bring the frequency back to where it was prior to the trip. So, a tie line carrying 1100MW into the system has to be covered the same way as the nuker. Further, the reserves that are going to cover these contingencies have very specific requirements. The NERC definitions and requirements for operation of various Power Pools can be found here:
http://www.nerc.com/files/ivgtf2-4.pdf
Different pools vary slightly in their definitions and method of calculation of reserves but all arrive at the same coverage in the same time frame.
Trips of lines internal to the system do not, as a rule require reserve coverage since, unless there is load loss associated with the trip, the system load remains the same as before the trip. The only thing that happens is the power flow within the system and perhaps on the ties lines changes depending on the proximity of the tripped line to the tie.
Ramp rates for units are dependent on the size and age of the unit. Modern units using computerized control systems have higher ramp rates than older manually operated units. A large unit is apt to have a number of ramp rates over the span of its output. These are determined by a number of things including auxiliaries such as feed water pumps, coal mills and feeders and other equipment that has to started or stopped, depending on which way the ramp is going. Hydro is very responsive in both directions since its only input is the amount of water that is put through the turbine. Taking into consideration only the ability of a unit to ramp and assuming a fully warmed turbine and that it is not near the end of core life, a boiling water nuclear reactor is as responsive to load changes as hydro since it changes load by moving voids(steam bubbles). All fossil fueled boilers, the determining factor in ramping providing the turbine is fully warmed, are capable of ramping fairly rapidly in the middle areas of their power curves.
There are other limiting factors such as minimum run times and minimum down times that may limit the availability of units to either start up or shut down that come into play in deciding on the optimum generation mix for a day.
