I'd say at least one Damage Up (1 Main or 3 Sub) is a must for both. It helps the Inkbrush splat more reliably, though ultimately you shouldn't be offensive with the Inkbrush (I'm not a fan of the Nouveau in that way) so it's just there when the opportunity presents itself. It helps improve Burst Bombs for the Slosher, which makes the Slosh-then-Burst/Burst-then-Slosh combo more reliable, and will let your bombs score splats more often.
Though the Inkbrush and Slosher are especially interesting weapons to learn, their Subs are also really important, and not so easy to master. The Sprinkler has an obvious use for coverage, but it can be used very defensively with some clever placement. The Burst Bomb is great for checking enemy turf for hiding opponents; the range is really big, even if the damage is peanuts. I'm still learning the Ink Mine, though I bet it could be the Inkbrush Nouveau's best chance at getting splats.
I like Bomb Range Up for the Slosher, to get those opponents that out range you off your back for the moment. Sub Saver for your Sprinkler is very helpful.