Hi,
You can do constant slow motion and constant fast motions in combustion.
All you need to do is load in your footage. Select the layer and expand it so that you see the footage operator. Select "Footage" and you will see two buttons, "Source" and "output". Select "Output" and on the right side you will see the speed controls. So you can speed your image up to whatever speed you want by adjusting the percentage or duration.
There is an option called frame-blending which is probably what you're after. Just turn it on and combustion will interpolate the frames.
For speed changes of the clip over time you can use a plug-in with combustion called "Twixtor"...
Hope this helps...
Splice...