It looks like it's possible to get multiple of most common monsters - are there any that this isn't the case for? Are there any I need to be careful not to waste the limited supply of on infusions, or any that can be missed entirely?
As far as i am aware there are no monsters you can miss in the sense of never being able to get them after a certain plot event.
There are 10 monsters that you can only get once. The first 3 you get by fighting. The other 7 by getting their crystal with improved Moogle throw. Here's the list: