While Genestealer Cults are unlikely to ever gain access to Planetary or Strategic scale weapons, they often engage in numerous acts of sabotage which can create the same types of effects before a battle. Thus they can still use the Preliminary Bombardment rules if the scenario calls for them.
If Detachment rules are in use, the Cult force can still only have a single Magus and a single Patriarch. You can never have more than one of each of these in the entire army, no matter how large.
Genestealer Cults have a Strategy Rating of 1D6-1, and a Vehicle Leadership Value of 7 for campaign use. For Missions involving Sentries, Cults use 10 Brood Brother Initiates (Initiative 3).
Fighting against Tyranid Forces – though not common, it is possible for the natural chemical or electromagnetic peculiarities of a planet to produce a mutated Cult that is resistant or not-effected by the Tyranid Hive Mind. Thus it is possible to have Cults fighting against their newly arriving Tyranid masters! All normal rules apply for these combats (though the Hive Mind is probably very puzzled by what is happening!). By the same token, Genestealer Cults can fight against each other (each being sure it is the true Cult, and the other a fraud).