I wouldn't call Halo Wars canon...
I know it was said somewhere that not all of Halo Wars should be considered canon. Not sure where though.
EDIT: This is all I could come up with. Its from Halopedia, but still.
Significant changes to the Flood's general existence were necessary due to the change of genre from first-person shooter to real-time strategy game. These are as follows.
Many new forms, such as bomber forms, were created for Halo Wars to expand the "roster" of Flood units. The official explanation is that these forms were created from creatures indigenous to Shield 0459.
Infection forms no longer push back the heads of their Elite victims; it appears as though the Flood actually use the hosts' heads. In addition, combat forms have more tentacles than before; they resemble spikes rather than flexible appendages.
Grunts and Jackals can be converted to combat forms.