From working on the ATAP as well as the Spec OPs units I have been doing I have learned a few things that I was always told shouldn't or couldn't be handled by XSI/Pandemic as far as animation and enveloping is concerned.
Enveloping.
Was always told you should only envelope to one bone.
For somethings this is a good rule to follow but they are mostly mechanical type of animations, walkers, flyers, doors, animated props and the like. For units though they will deform better in game if some of the vertices are enveloped between two bones/ Legs and shoulders especially.
Using the smooth tool in the enveloping paint tool is great for this.
Helpful hint for enveloping: Put a key on all the bones on the first frame, with this you can then move, rotate or whatever the bones to see how the envelope works in certain positions and adjust the weight blending so it looks good. To get back to the base pose just move the timeline slider and it will go back but the envelope changes will stay. Important make sure "AUTO" is off on the animation tool bar.
Animation:
Linear vs Spline:
Was told all animations should be made linear.
Once again this really depends upon what you are animating. Some animation only look good if the animation is changed to linear. But Splines work just fine in game and make for better animations.
IK vs FK Animation.
Once again the understanding was all animation needed to be FK to export properly.
Truth is they can be either one or the other or a combination of both. I used a combination of both to do the death animation for the ATAP.

Incidentally it was a fluke that the music worked out as it did. And yes some of the animations aren't done yet.
Make sure the FK box is checked on the animation export.
Why use IK over FK for animation, it is a lot easier to do poses and movement using IK animation and if you use constraining objects and set a neutral pose on them it is even easier because your constaining object will have a set of neutral numbers of 0 which makes it real easy to go back to the beginning.
Will add more to this as I remember or find out more things but just wanted to at least get a start on thing.
The help files for XSI as well as websites listed in the FAQ are still the best source of information on how to do things in XSI.



