A Cult Leman Russ has the
Tyranids keyword and is thus eligible for any
Tyranids Crusade force. Since it also has
Genestealer Cults keyword, it can also be included in any
Genestealer Cults detachment, subject to the rules for a
Brood Brothers unit.
Also note:
GSC Codex Errata wrote:Crusade
You can add Astra Militarum units to your Genestealer Cults Crusade Force using the Brood Brothers rules presented here. However, that Astra Militarum’s <Regiment> or Militarum Tempestus keyword is replaced with Brood Brothers at the point that it is added to your Order of Battle, instead of when mustering your army.
When mustering your army, these Astra Militarum units cannot include any other Genestealer Cults units that do not have the Astra Militarum keyword in their detachment.