Do yourself a favor and research MSI's afterburner graphs to understand what your temps are really. Most of the time what the 100% power limit is talking about is the Power Target, the Temp Target shouldn't be higher than 84c, moving it down will actually undervolt the card, you should aim to keep the cards below 65c, if they go above you might be losing some hashrate(Depends on the cards, but I've had it confirmed for my cards).

Also do yourself a favor, stay away from CPU mining in general, its not as profitable as it was back in the day. Now adays its literally horrible! Also stay away from MinerGate(Keeps your funds on their end, and sends you it when they feel like it, plus their software is clunky and not optimized for mining in general)

In all honesty, if you want to avoid Fire situations, learn how to build a computer from the ground up first, make sure you learn the ins and outs of it and make sure to use GOOD QUALITY components, and don't for any reason skimp out on any Power supply or wireing.... its basically a fire waiting to happen.(Or even better just hire someone to help you I'm sure there are people out there that will build you mining rigs for a price)

But if you have to come out on here asking what will start fires for pc's and if its safe to run overnight, I guess you haven't done much research on google(Or your favorite search engine) or on youtube about mining 24/7. Some videos and guides tell you about the safeties of mining 24/7.