Yep, you can. They are Allies of Convenience. What this means is there are very few penalties/restrictions to fielding them together. The only down sides to Allies of Convenience that I can think of is that they cannot use each other's Transports (no putting Genestealers into Valkyries, for instance), and any Psychic powers that targets "enemy" units (like Nova spells) will also target your AoC Ally. I also believe you can't use Blessings on them either, but IDK about that.