They like tunnels and things they can hise in. when mine is outside he has some drainage pipes in his run which he likes to run through. They also need something to gnaw on to keep their teeth trimmed down.
Not sure as have only ever had a mixture of rabbits and guinea pigs, as they make good company for each other. If you breed them you must seperate them when they are weaned to prevent more little ones. Have been told this on good authority from someone who breeds them (intentionally).