[ATJ]

From Shimek's article:

These final two groups of isopods, the Cirolanids and Aegids are so similar in appearance that it typically takes an expert to distinguish them on the basis of a few minor structural details. Not being an isopod expert, I will treat them together. These are flattened, streamlined crustaceans with a smoothly rounded or tapered and pointed front end. Their very large eyes are found laterally on the head and may occupy as much as half of the space of the head. They are strong, very fast swimmers and have no obvious trailing appendages. The thoracic legs are robust and end in very sharp gripping claws. Aegids differ from Cirolanids in several small details; probably the most evident is that both pairs of antennae are often evident and visible, IF you can find one that holds still long enough to observe that trait.
 
The taxonomic Family Cirolanidae is huge, consisting of several dozen genera, and probably many hundreds of species. Many of them are generally benign animals that are obligatory carrion-feeding scavengers. Some of these scavengers have been found in aquaria, and appear to be very well adapted to being part of "the clean-up crew." Many of the rest appear to be capable of scavenging when such food is available, but they will occasionally swim up into the water and attack fish, fastening on and sucking their blood. Finally, several species appear to be more-or-less obligate blood-sucking parasites of fishes, although some may be able to live for extended periods by scavenging some dead food. Within the last couple of years, some of these latter species have been seen in aquaria with alarming frequency. In many cases, these infestations appear to be the result of a pregnant female that enters the aquarium and then drops her brood of 10 to 30 young, all of which are immediately hungry for a nice meal of fish blood. A hobbyist will see the alarming sight of one or more fish with from one to twenty blood-sucking parasites on it. Often the isopods are nocturnal, and unless the aquarist is alert, they may not notice the parasites, as the bugs drop off the fish shortly after the lights go on and find shelter in the rocks. Prolonged exposure to such densities of blood suckers WILL kill fish.

The problem you have here is determining what species you have. Do you have the relatively benign "obligatory carrion-feeding scavengers"? Do you have scavengers that will occasionally latch on to the fish and get a meal? Do you have the species that are "more-or-less obligate blood-sucking parasites of fishes"?

The only way I think you will be able to determine is to put a sacrificial fish in the tank, as suggested below. If the isopods leave the fish alone, it is more likely they are obligate scavengers and would make a good addition to the tank. If the isopods attack the fish then you will need to remove them, and the sacrificial fish is the way to do it. As suggested below, choose something that is easy to catch.

The only way to rid an aquarium of these animals is to catch them all, which although tedious, is possible. Generally, this involves using a sacrificial fish, usually something easy to catch and moderately large. A yellow tang is a good choice for this because the fish's color pattern allows easy determination of the presence of the parasite. The other fish in the tank are collected and removed to a quarantine tank, and the "bait" fish is introduced. This fish is checked periodically and, if the parasites are seen on it, it is netted and removed to a flat surface where it may be immobilized with a wet paper towel. The parasite is removed with a pair of tweezers or forceps. The fish is then returned to the tank, and the procedure repeated. Often the parasites are nocturnal; consequently, the fish will have to be examined before the lights come on. The aquarium is probably free of the parasites if none are seen on the fish for a month or so after the last one has been collected.

As to the remaining live rock, in my opinion, it isn't going to make a whole lot of difference if you take it as you already have the isopods in the tank. The sooner you take it, the better, as you can deal with the isopods in one go, rather than cleaning up the tank and risk reintroducing them later.