The numbers show their resistance to that damage type, so higher numbers means a higher resistance just like they would on you.
And yeah, not all enemies have all resistance types. Most only have ballistic and energy resistance, then the third most common is radiation resistance - but very rarely you'll see poison resistance, too.
Well, radiation damage works really differently compared to other damage types. Basically, 100 Rads is always 10% of your health no matter what, and most nonhumanoid enemies are outright immune to radiation. Plus, you'll have a hard time finding weapons that deal radiation or poison damage. The Gamma Gun is the only one dedicated to irradiating enemies (it can be modded for more standard damage, though), and the only time you'll find a poisonous weapon is from legendary drops. But for legendary items that deal either poison damage or radiation damage, it's best to mod them for the best rate of fire rather than raw damage IMO.