Even the most sensitive aquatic plants are usually not bothered by 1 teasp (6 grams) of salt per 10 gallons, which is about 150 mg/Liter (60 mg/L Na + 90 mg/L Cl-). The amount you'd be adding via ion exchange is way less than that. And a planted tank probably doesn't have much if any excess nitrate to be removed anyway.
BTW, nitrate is a heavier ion than chloride (NO3 = 62 amu; Cl = 35 amu) so each 1.00 gram of NO3 removed would correspond to 0.56 g of added Cl-.
The 3.1 mg/L "tolerance indicator value" for brook trout in the Meador (2007) report was based on chloride field measurements at sites where brook trout were collected, NOT on the physiological tolerance of brook trout. Clean, clear headwater creeks, where brook trout usually live, have low chloride, but they CAN live in much higher salinity. There are even sea-run populations of brookies! Here's a link to the Meador paper, which the URI paper references: https://sites.biology.colostate.edu/...02007%20EI.pdf
The URI paper also mentions 160 mg/L chloride affecting salamander larvae in vernal pools. That's probably a more meaningful estimate of harmful chloride levels to sensitive freshwater species.