I used to run a similar team and I have health and defense mods on Nihilus with primary stats of speed arrow and potency cross, and lots of speed secondaries for a total of +97 speed.
In my opinion, with Palp lead and Sion, Nihilus gets enough speed boost already so that speed mods aren’t needed—if you can get significant speed on secondaries. Potency matters to help drain force hit when enemies aren’t debuffed.
What I also like about my setup is that if I use Nihilus in other squads or as a leader, the health and defense are helpful and don’t require re-modding.