Umm...not sure how it affects summon undead. For Animate Dead and an evil Cleric's ability to command undead, you're limited in the number of HD of undead you can control (or create as well, in animate dead's case). A skeleton human warrior has 1 HD. A level 2 evil Cleric can command a number of HD equal to his cleric level, so he could command 2 human skeletons at any one time. (A cleric needs to have 2x as many levels as the undead creature has HD to command it instead of rebuking it, which is why I made the sample cleric level 2).
Likewise, a level 10 Wizard using Animate Dead can control up to 4x his caster level (CL) of undead total, and can animate up to 2x his CL as undead with any single casting of the spell. That's 40 HD total, and up to 20 HD raised to unlife in one casting. Say the party slaughters 5 Tigers (8 HD) in combat, just as an example (quite a feat to accomplish). If the Wizard has no undead minions at the moment, he has enough of a HD pool to animate and control ALL of those Tigers as Skeletons (8 HD x 5 = 40 total). However, he can only animate 20 HD per casting, and you can't "half animate" something with extra HD potential, so he can only raise 2 per casting (16 HD; 3 would be 24 which is too much). If he wanted some zombie tigers, then he's even more limited: creating a zombie doubles the base HD of the creature. This is why you can't create zombies out of anything with more than 10 HD, since Animate Dead states you cannot create any single undead with more than 20 HD. Or if it doesn't, the skeleton and zombie entries say it anyway. So...Zombie Tigers would be 16 HD. Our Wizard can only control two of those total (32 HD), and can only create one per casting. However, he has enough HD left over to create a skeleton tiger with a third cast (32 + 8 = 40).
Note that using a desecrate spell on the area is a method to increase how many HD you can raise in a single cast, but the cap on how many you can control remains the same. If you exceed your control limit, you lose control of some of the undead you created, starting with the least recently created, I believe. You can always choose to release control of certain undead before raising more, so you can decide which to let go of.
As far as whether it's metagaming to know exactly how many HD each creature would be as an undead...I don't know, it's up to the DM. It's worth noting that you need 25 gp in Onyx for the spell per HD of undead you're raising, so the characters must have some way of determining...I'd rule it a Knowledge (religion) check, or a knowledge (whichever is appropriate for the creature when it was alive) to determine how much onyx you'd need.
Hope I didn't just confuse you more with the long reply.