In general it's always toxin. You can make corrosive or viral with just one mod.
For the nukor specifically, magnetic is best for a priming build as it allows the most statuses at once. Heat iirc is best for dps.
For the plasmor I hear you can't go wrong with magnetic either. Apparently the galvanized status mod does some funky shit and makes it stronger? Toxin is a safe choice though, helps open up a slot for a qol mod.
You don't need Slash for priming though. As a Condition Overload primer, it only matters that you have a status, not what that status is. So Magnetic + Cold + Radiation is actually better than Slash + Impact, if you intend to use the Nukor to prime.
The only exceptions are Viral, which you always want a lot of, and sometimes Heat (if you're doing a heat inherit build.)
If you want to use the nukor as a main damage weapon and not a primor, hemorrhage/slash is okay, but the chance of getting a slash proc is so low that you're better off with a generic build.
What I mean is, if you want to spend your progenitor element and mod slot to get two statuses, you can do better than Impact + Hemorrhage. It's a small chance stacked on top of a small chance (since even with a 60% progenitor your elements will be diluted on a primer.) Going for something like Magnetic + a Heat 60/60 is just more efficient for priming, because you get status chance and the heat isn't contingent on getting a magnetic proc.
(It doesn't help that many of the best melee weapons already proc slash.)
You should run Magnetic (Lich), Radiation (innate), Viral+Heat. Since secondaries have Primed Heated Charge, focusing on Viral+Heat is better than on Impact and pseudo Slash. This way, the weapon can handle any enemy type.
Using Hemorrhage and similar is a valid option only when the does a guaranteed Impact proc or is too slow to reliably apply a lot of Heat stacks.