Yeah, the summon is completely dependent on the fame of the dead creature. The HPs of that summon has no bearing on your spirit speak.
I really like the summons... but only when the monster is of such high fame that I get a skeletal dragon. Aarons is right that most of the summons are fairly weak. I would say all are weaker than an Earth El summon, except for the Skeletal Dragon. The Skeletal Dragon is quite powerful... though they do loose hit points over time and will automatically die eventually. I've been known to be running through dungeons with 2 x Earth Els, 1 x Shadow Wisp, and 3 x Skeletal Dragons by my side. With that team, it works pretty well, but you have to constantly be killing things with high fame to keep animating new Skeletal Dragons.
BTW, animate dead will kill your karma, because you have to keep summoning new ones. So it's arguable whether it is compatible with your character with Chiv. It really depends how often you use it. It could be handy, however, in some situations where you need an extra buddy on your side.
One note -- I've noticed the Skeletal Dragons don't attack on their own. They tend to wait until I make the first strike. Once I target the foe, they dive into the fray. So I usually hit with an explosion and then ivis before the monster targets me. That seems to work.
-OBSIDIAN-