I'm adding a second battery and want/need to get a battery isolator.  The battery is an XS XP750 AGM Battery Power Cell and I'm getting it because I have a 160 amp alternator, NVX JAD 800.4 and Rockford Fosgate T-1500bd.  That RF amp pulls 180 amps max (I don't typically listen to sine waves) so I need a 200 amp isolator, correct?  Would there be any harm getting the 500 amp isolator because (believe it or not) I can find them cheaper (and in stock) than the 200 amp models.

What style of isolator are you looking at? Some styles have voltage loss, which will be greater the larger they are.  

I was looking at the NVX models, I've never used an isolator but did some basic research.  The NVX appear to be a diode based and not solenoid based.  I don't mind a .4-.7 drop in voltage, I'm not competing and should be fine with a 160 amp alternator and strong primary battery.  Oh, I'll be running the RF at 2 ohms so it won't be putting out more than 1200 watts RMS.

 You pretty much just answered your own question. The smaller one will be closer to the .4 drop and the bigger one closer to the .7. If that don't bother you, there is no other reason not to get the bigger guy.