I was one of the first to mention that making this module a high slot item would be trouble for the Helios and Blockade runners, even before it was turned to a high slot item after some people repeatedly asked for it: Nullification and WCS Updates – testing has begun! - #816 by Gerard_Amatin
But even with that thing as a high slot item, I see no issue with the Helios as a nullification module is not always needed. High sec and low sec exploration has no need of this module and neither does combat probing have need for this module. Which means the slot layout of the Helios is better than that of the other ships in those scenarios.
Were it that the Helios was worse in every possible scenario than the other T2 explorers, I would see an issue. But right now each of the T2 explorers has things they’re good or better at than the others. And that’s perfectly fine.