Adaptive equalizer for IIR channel?
Note that the inverse of an FIR system is IIR, and the same is true for the inverse of an IIR system, unless it is an all-pole system, the inverse of which would be FIR. So in most cases the ideal equalizer should have an infinitely long impulse response in order to perfectly invert the channel. In practice almost all adaptive equalizers are FIR filters which can only partly compensate for the channel response. This is why I think that there should be no reason for your equalizer to perform worse for IIR channels than for FIR channels. You could post some more information about the channel responses and the convergence of your equalizer to make it easier to see what's going on.

As for the zero forcing criterion, such criteria for designing an equalizer only make sense for noisy channels, because otherwise there is no trade-off between equalizing the channel response and noise enhancement. Zero-forcing and MSE criteria result in the same equalizer in the absence of noise. If you have no noise, the only problem is the (hopefully linear) distortion of the channel and naturally this is the only thing the equalizer would compensate for. However, with an FIR equalizer exact zero-forcing is usually impossible as discussed above.