friggin gross.....i would quarantine all the fish in a seperate tank and bleach every inch of the infested tank.including filters,etc...
From http://www.eoearth.org/article/Crustacea regarding the life-cycle of Isopods, which is the order that these things belong to:
So, it's possible that this thing had already found a male and mated before it got into your tank. It's also possible that it was gestating the eggs or newly hatched young at that time and that it released them into your tank before you found it...although I would think this is highly unlikely. The rest of your fish are probably just fine.
What I find interesting is that everything I found regarding this particular parasite refers to it infecting a rose snapper, which is a saltwater fish. Is this somehow related to your "red tail sabertooth"?

For the love of g-d and the good of the MFK community please go scorched earth on these horrible little sob's.
I'm sure It would be greatly appreciated by all if you continue to keep us in the loop. Your willingness to share all the info relating to your loss may wind up saving someone else's tank.
Ive got nothing to add in terms of treatment options; every idea i had has already been suggested. I do know a hobbyist who has a masters degree relating to wet ecosystems & I will call him and have him look through the posts to see if can offer any advice