You can limit your torque output by limiting your fuelling at certain points in the rev range within your ECU, if you talked to your tuner I'm sure they'd be able to do it, on diesels increasing the amount of fuel injected into the engine is how you accelerate it,
as for going straight to the GT2560 on it's own, I wouldn't think that it would suit your engine when run on it's own, the GT2560 flows up 340~hp worth of air, but does so at a relatively low boost pressures, your standard turbo is the other way around, doesn't flow as much air but can run high boost pressures in order to force that air into the engine (if it needs to do so, all applications are different) a diesel engine like yours would need a fairly high amount of boost to run some decent power (again its a relevant thing, how much power is needed/wanted is up to you)
The reason for suggesting a compound turbo system (not twin turbos since the 2 turbos aren't the same) is that we can exploit the benefits of both turbos by running them in there optimal range, by that I mean that gt2056 makes lots of boost, and the gt2560 moves lots of air(and another reason for going compound is that you retain much better drivability than just "wacking a big turbo on it" ), the thing is with turbos is that they will compress any air that you give them, even if the air that's feed into the inlet of a turbo is already pressurise above atmospheric pressure, the output air pressure is multiplied though rather than just added together...
for example,
lets say both of those turbos are running 1 bar of positive pressure each, (now thats 2 bar of absolute pressure why? because atmospheric air pressure at sea level is 1 bar or 14.7psi so if we increase the pressure by 1 bar that gives us 2 bars of absolute pressure or 1 bar positive pressure, absolute pressure is what is displayed on a turbo compressor map)
so both turbos are running 1 bar each which is 2 bar absolute each, so we multiply them to together as 2x2 which equals 4 bar absolute pressure take away 1 to calculate positive pressure(inlet pressure) and you get 3 bar or 45~psi,
the point being is that neither one of the turbos mentioned would be able to do that without throwing out loads of hot air,
so compounds will make the boost while keeping both turbos working efficiently, you'll have lower exhaust gas temps since have lower exhaust back pressure and make as much power as a large single without sacrificing low down torque
hope that helps, I know it's a lot to read :S
Brett