Do action potential thresholds vary in "capacity" significantly, and if so, does input frequency correspond to action potential thresholds? To clarify, I'm asking if the level of stimulation / polarization required for a neuron to reach action potential / excite the neuron varies significantly from neuron to neuron- even differing in neurons of the same type. 

And if so, I'm curious to know whether the number of the neurons feeding into a neuron (and the frequency by which they normally fire) corresponds to the level of stimulation required for excitement. I'm wondering if neurons' thresholds for action have sensitivities (levels of input required to reach action potential) based on the number of neurons feeding into them in order to create a balance of input rate / excitement rate. 

To rephrase the question, I'd like to know if the action threshold of a neuron changes to fit the frequency with which it's excited.
The key to determining if depolarizing a neuron will result in an action potential is the threshold potential of the cell. The threshold potential is basically the voltage, around -55 mV, at which the Na+ channels open. A number of factors (covered in Wikipedia) affect the exact threshold potential.