It probably won't slow down your cpu fan - unless you’re using dozens of fans. Many people think adding fans will help cooling, when in fact it can hender cooling. The air flow should be laminar when going through the system - typically entering at the front bottom and exiting at the top / power supply - so the heat flows out. Adding fans helter-skelter can distrurb this flow and hender cooling.

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No, they’re run on voltage, which is regulated through your psu and motherboard. You can adjust these settings and increase fan speed in the summer.

To access bios press Del, or whatever to get into it at the begin up screen. Once entered, you will see a section for status or pc health. You can see current temps sometimes, and also adjust fan rpm or % at certain temperatures. So as the cpu gets warmer, the fans spin faster. =)

Also you can try using speedfan for fan controls if you can't find it in your bios. And monitor temps.

http://www.almico.com/speedfan.php

Have fun.

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No because the electrics are designed for a number of fans.

I'v got four in mine. 1 for the PSU, 1 for the CPU and 2 chassis fans. Been running like that for a number of years and runs cool all the time. Case is cold all the time. CPU never goes above 32C!

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If your system is so underpowered that it's affecting the fans, I'd be more worried about the graphics card. They generally suffer the most from dodgy power provision.
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Yes, depending on your PSU. They take up power, and when you run low, the fan's slow! That's a tiny saying I just randomly created. I hope I helped!
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You could not fit that many fans to pull down the PSU.

A friend of mine has 11 fans in his Acer ally case with a 450 watt PSU and that runs ok

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Wouldn't have thought so, the cpu will take a lot more power than a few fans.
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Unplug some and find out?