Yes, the wireless keyboards are bluetooth, and until the OS loads the bluetooth kext, it's basically non-functional, so you need a wired keyboard around for these things.
There won't be a problem if you wanted to just leave the old wired keyboard plugged in all the time but just normally use the wireless one. Just set it somewhere out of sight. It eats up a valuable USB port, but it won't hurt anything otherwise so long as you don't leave something sitting on the keys of the wired keyboard.
And your other question about the external drive is yes. I tend to always install a disaster recovery copy of the OS onto an external drive. That way, assuming the internal drive craps out one day, I can muddle along until I have time to take the thing in for repairs. Technically I could do them myself, I even have a set of the suction cups needed, but Apple's repair system isn't set up for people like me.
Anyway, it's always a good idea to have a second OS install on an external drive for disaster recovery type purposes. Whether that's 10.6 or 10.7 you can decide. Just install the OS, then boot to it, install your basic set of apps, and then you can just leave it.
Another thought just occurred to me on a completely different note. Since you just got a new iMac, you might want to start system profiler (or system information as it's been renamed in 10.7) and check under the Serial-ATA section. Copy and paste the drive model number into Google, and if it's a 1TB Seagate then you may want to arrange to have your new iMac taken in. There's an issue with the Seagate 1TB drives that went into the 2011 iMacs where they fail at a much higher rate than normal. I'm not entirely up on all the specifics, but would probably be a good idea to get someone to check yours, and replace it if necessary. Apple's also offering free data transfer services for this one if you need it, and the drive is still functional anyway. Might want to make it next weekend or at least Monday, since Apple's repair site is down for maintenance this weekend, meaning they wouldn't be able to process your repair anyway.