Although it is always best to err on the side of caution, Hydroxycut Hardcore has 300mg of caffeine where as Leviathan has about 75mg (but also has quite a bit of Yohimbine). I couldn't say for sure how much caffeine is in NOxplode because it's listed as a propriotory blend, but I think it's around 100 (I actually have it in my head that it's about 70mg, but I'd like to play it safe) for a scoop.
I consider 300mg an absolute maximum for caffeine, but even that much is pretty sketchy. I would wager to say that NOxplode and Leviathan might make you a little shakey despite having less caffeine than HH + NOX, just due to all the other stimulants in it.