Error amp compensation on pwm controller
When in doubt, read the datasheet, apply some circuit design knowledge, and maybe go dig up some applications notes.  The SG3525 has become a standard part, so there may even be books written around it.

If you look in the datasheet, you'll see that the error amplifier gain is stated as a transconductance, with a range of \$1.1\mathrm{mS} &lt; G_m &lt; 1.5\mathrm{mS}\$.  Later on in the datasheet the frequency vs. gain characteristics of the thing are given (Figure 4 in the datasheet that I linked to).  The PWM section has thresholds specified (just look at the datasheet).

These three things together, plus your circuit topology and supply voltage, will give you enough information to approximate your power supply as a linear amplifier and calculate its open-loop transfer function; that, in turn, will allow you to calculate the closed-loop transfer function and assess stability.  That'll give you a first cut at the circuit behavior, at which point you'd be wise to do some simulation &amp; testing.

As a first approximation, though, if you hang a capacitor off of the compensation pin, the gain from the error amp inputs to that compensation pin (and, hence, the PWM comparator input) will be \$H(s) = \frac{G_m}{C\,s}\$.  In other words, the error amp will be a pure integrator.  I suspect, given the information in Figure 4, that for all practical purposes the action of the capacitor will swamp out the amplifiers inherent frequency roll-off, and it'll act like a pure integrator.

As for connecting it back to the inverting input through some feedback resistors or a wire -- sure, maybe that'll work.  It'll turn it into a voltage follower (or voltage amp if there's more resistance from that pin to ground), which means that there would need to be some compensation outside of the chip to make the loop work.