Regarding the chains, I wouldn't really find much worry in reusing a pin on an 8 speed chain that's being used on singlespeed. As long as your chainline is good, there shouldn't be any forces trying to push the outer side plate off the pin. Its a bit more risky if you're actually using it with gears, especially with a front derailleur.
8 speed chains have pretty minimal peening of the pins, less than 9-11 speed chains. Plus, when you push a pin like that it only removes one peened end, not both.
I'd be more concerned about the pitch of the new link being different (exactly 1/2" instead of the rest of the chain which is 1/2+a little") and causing problems, but only if the rest of the chain is pretty worn. If you're wanting to change cogs on your SS much at all, I'd just get a chain for one cog and another chain for the other cog. That'll make everything last longer too and easier for you when you wanna change back.
But a lot of chain tools, like many of those made by park, have a stiff link remover part on them which is just another tongue that you place the chain over, but this one is closer to the screw and doesn't have the chain touch the anvil. Thus putting the chain here and applying pressure to the pin will tend to spread the outer side plates apart as opposed to them getting pushed together like when pressing the pin back in.
Personally I'm just using 3 singlespeed NX-01 chains on rotation. Soon I should be able to make a new chain from all the spare links from previous chains. I don't change my cog ever.