[Suicide Junkie]

For the nuclear stuff:
Why not have Nuclear Missiles for use against ships, and Nuclear Bombs for use against planets.
You can say that the missiles are built lightly, and can't withstand an atmospheric entry (insignificant effect on vaccuum planets, since there is no atmosphere and the bLast dissipates into space)
The bombs would be designed for planetary assaults, detonating in the lower atmosphere, or on impact with the surface of a vaccuum world.

You can then also make the nuclear warheads do a different amount of damage to planets vs ships.