Will Krylon work if I spray it over cured cerakote, or will it come right off if exposed to rain / sweat / etc?  If so, anything in particular I need to do to the current cerakote finish before I spray over it?

I have had my rifle cerakoted H227 Tactical Gray for about 2 years now. The color is just a tad lighter then I ultimately want, being just solid tactical gray. I would really like to lay some fish net on it, and spray over a light black mist, maybe a different shade of gray, etc.

It should stick, but like any Krylon it will wear off with time.

Maybe try a dab of pain in a spot you wont see, like remove the hand guard and try there and see if it sticks.

I cerakoted several parts over the weekend, and mistakenly used too much hardener thus creating a glossy finish.

Okay, I thought, I don't have enough to refinish it, so I'll use 0000 steel wool. Worked somewhat, but I got greedy and wanted more.
So I took some krylon flat clearcoat and sprayed it. the slightest nick would damage it so I had to try to remove it, which adversely affected the look of my glock lower (not the metal parts). The stippling was the worst!
A day after it dried the one piece I have still kylon'd is more durable, but can still be scratched off with a fingernail. I know that it's just clearcoat, but I wouldn't do it.

Aervoe, maybe. Krylon.... I wouldn't. just my .02

Cerakote is a ceramic finish, and is designed to chemically repel everything, including itself. (IE, you can't cerakote over cerakote once it's been cured fully)

So yes they krylon will go on, but will it stick to any reasonable expectation? Likely not.