Is it possible to place HS22 and LS22 blades in the same IBM BladeCenter without compatability issues?
Yes, you can mix any combination of blades in the same BladeCenter chassis provided the blades themselves are supported in that chassis.
The table here lists the compatibility table between blades and chassis.
(huh, actually the LS22 is no longer listed. You can find it's compatibility list here.)
On IBM's webpage there is a "bladecenter redbook" this correctly answers your question.
This is one of that: http://www.redbooks.ibm.com/Redbooks.nsf/RedbookAbstracts/sg247523.html