Its not likely that a high calcium will be "toxic" to anything, so no... I do not think it will kill anything. I personally have never killed anything from high Ca and I have overdosed before when I first started out.
What does happen is this... Calcium getting too high will cause precipitation of excess calcium. This brings the calcium back down, BUT it also brings down your alkalinity.
https://forums.saltwaterfish.com/t/274631/ph-and-alkalinity-unmasked
This is an article I wrote on alkalinity and how it relates to pH.
From this you will see that lowering your alkalinity can have a nasty effect on pH. Dropping your pH fast is definately bad, and YES this can kill fish.
Aquarium Pharm makes an alkalinity test kit too. Fairly cheap, also. Not the most accurate, but its more than capable of giving you all you need to know. (Its more than accurate enough to know if you are low or high out of optimal... and thats all you need to care about)
The recommendations given to you are very sound, IMO. What they are telling you will work.
One more thing. Are you using tap water or non-deionized water? If you are, its very probable that there is calcium in this water. Mix it with a high calcium salt like Oceanic and you could put yourself over 500.