Depends on both the tetras and the killis.
For tetras, I can highly recommend ember tetras. They are slightly smaller than neons, slow swimmers (although they will shift pretty fast if they need to) so their speed won't stress a betta, and neither I nor my friend have found them to be nippy. :)
There are a heck of a lot of killi species out there - more than 1000 - so a source saying "killis are peaceful" is making a very sweeping generalisation. It really depends on the species.
Typically, bettas aren't kept with killis. I've recently (as in last Tuesday) added a pair of locally bred male fundulopanchax gardneri gardneri to my sorority and so far nothing has gone wrong, but as they get more confident it easily good. They also have mouths the width of their heads, so they may eat smaller fish (I fear for my ember tetras if my killis grow much more!)
I'll be able to give a better recommendation on them as betta tankmates in the near future, I hope. :)