is it alright to have temple guard of the last genertion and old army book with this newest one? also have them together guarding a slann ?
Yeah there is nothing wrong with using OOP models, they can look a bit out of place in mixed units but its perfectly legal and quite often can make the army look unique and better.
Yea it's totally fine. A lot people have the old 5th and 6th edition models, and use them in the place of the new ones. I mostly use 6th ed. models for conversions, but they can still be used.