If the moon was large enough to hold an atmosphere, then in principle there is no reason that it could not be capable of being terraformed. There would be some interesting complications, however.
First off, a moon orbiting a Gas Giant would likely be tidally locked with one face towards the primary. Days would be very short, but the hemisphere facing the primary would be illuminated by the primary itself, as well as the primary and the sun during part of the orbit, and in darkness for a very short period of time. The side facing the primary would have the "hot pole" where the primary is at the zenith, while the opposite side would have the "cold pole", so atmospheric and hydrospheric circulation and heat flows would be dominated by this.
The leading hemisphere of the moon would be bathed by the energetic radiation trapped in the primary's magnetosphere, while the trailing hemisphere would be relatively shielded. The interaction between the energy deposited from the hot pole and the "leading pole" could be defined as a series of concentric bands at 90 degrees from each other, leaving the moon covered in a sort of checkerboard of ecosystems based on energy inputs.
Depending on the numbers of other moons, the core of this moon might be "kneaded" by multiple and overlapping gravitational pulls during its orbit, making the moon quite active tectonically. Lots of volcanoes and active plates would make the surface quite active, as well as subducting lots of water and carbonate rocks. The hydrothermal and carbon dioxide cycles on this moon would be much faster than on Earth.
Since we are in the middle of a very deep gravity well, you should expect the moon is also subject to lots of collisions with asteroids and comets. This would add lots of water to the moon, but also reset ecological "squares" that were hit, meaning evolution would be going in fits and starts if the moon already had or was seeded with some sort of "native" life.
IF the Primary was tipped over like Uranus and the moons were in orbiting facing the sun at all times, then the positions of the "Hot" and "Leading" poles would be different. There would actually be a third pole, where the primary is overhead at all times, with the sun coming down at a high angle but also being permanently illuminated by the primary as well. There would be no diurnal cycle as we understand it, but the solar and hot poles would always be illuminated, while the dark pole (opposite the solar pole)would be in darkness and the cold pole would only have solar illumination, but more constant than the cold pole of the first example. Trying to trace the energy flows in these moons would be very interesting indeed.
