EDIT: After sleeping on it I figured it out this morning. I had put the entity in the node path, not the entity group 
I am trying to exclude any "fresh" zombies from DarksZombies from appearing in random spawns:
<csv xpath="/entitygroups/entitygroup[@name='ZombiesAll']/text()" delim="\n" op="add" >
zombieParasite
zombieFatParasite
zombieExperimentZ
zombieArmy1
zombieFreshFemale1
zombieFreshFemale2
zombieFreshFemale3
zombieFreshFemale4
zombieFreshFemale5
zombieFreshMale1
zombieFreshMale2
zombieFreshFemale1Feral, .25
zombieFreshFemale2Feral, .25
zombieFreshFemale3Feral, .25
zombieFreshFemale4Feral, .25
zombieFreshFemale5Feral, .25
zombieFreshMale1Feral, .25
zombieFreshMale2Feral, .25
</csv>
I've created a modlet, (which loads after DarkzZombiez) and tried the following lines of code:
<csv xpath="/entitygroup[contains(@name, 'zombie') or (contains(@name, 'Fresh'))]/text()" delim="\n" op="remove" >zombieFresh*</csv>
<csv xpath="//entitygroup[contains(@name, 'zombie') or (contains(@name, 'Fresh'))]/text()" delim="\n" op="remove" >zombieFresh*</csv>
<csv xpath="/entitygroups/entitygroup[contains(@name, 'zombie') or (contains(@name, 'Fresh'))]/text()" delim="\n" op="remove" >zombieFresh*</csv>
<csv xpath="//entitygroups/entitygroup[contains(@name, 'zombie') or (contains(@name, 'Fresh'))]/text()" delim="\n" op="remove" >zombieFresh*</csv>
...but I am still getting "fresh" zombie spawns in the wild.
If anyone sees the error in this I'd really appreciate it!
Had a sleep on it and discovered it in the morning.