The 6.1.3-6.1.5 jailbreak was only a real target as an end-game jailbreak for the new bootrom 3GS on 6.1.3, and the iPod Touch on 6.1.5. iPhone 4 users with iOS 6 blobs can also choose to use the 6.1.3 jailbreak if they did not save blobs for a working Evasi0n iOS.
Exactly. Though none of the tweaks in Cydia are ready for the iOS7 jailbreak yet. So you'd have nothing really to 'play with' on iOS7. Where jailbreaking in 6.1.3 opens you up to a world of things you can use immediately with no problems. Just remember.. once you update to iOS7 - you can NEVER go back to 6.1.3 again.
On A5+ devices yes. Remember that the iPhone 4 is the last device to openly support downgrades back to iOS 6 in the iOS 7 supported string of devices, so readers on the iPhone 4 on iOS 7, you can downgrade if and only if you have blobs. All other users, you cannot downgrade from iOS 7 to iOS 6.
Cool just have to find the how to guide.
The tool itself shouldn't be hard to navigate. Remember that p0sixspwn is a
userland jailbreak, just like Absinthe, Evasi0n, JailbreakMe.com, and SpiritJB, so following on-screen instructions will do the trick. It's not as time consuming or lengthy in steps as say Sn0wBreeze, which requires you to download the IPSW, have it customized, then restored using iTunes, then booting tethered afterwards, etc. I haven't had the chance to get my Macbook back to test it out so I can test it out with my iPod Touch 4th gen, but it should work fine considering @iH8Sn0w and @winocm are vigilant in pressing out issues at the speed of light as shown by the p0sixspwn untether package in Cydia on release day, which went through more updates than Evasi0n6 did in its entire lifetime, all within 6-8 hours of the p0sixspwn release.