Unfortunately, the characteristics of nuclear weapons change drastically in the vacuum of space. This is one of the things the show got Very Wrong.
First, blast disappears completely without an atmosphere. This means there's no shockwave, which typically accounts for 40%-60% of a weapon's total energy.
Second, thermal radiation also disappears. There is no air to absorb the high frequency radiation and convert it to heat. Thermal radiation typically accounts for 30%-50% of a weapon's total energy.
Third, because there is no atmosphere to attenuate the radiation, it travels much further and diminishes only in proportion (inverse square) to the distance from the detonation.
What you essentially end up with is a lot of gamma rays and neutrons, for which we can assume most ships, especially those of the First Ones, have a high degree of shielding. No BOOM. No fire.
Sorry to disappoint.