Been years since I've read FM: WC, but didn't the Vipers also use conventional infantry in some capacity?
They did. All trueborn warriors, too. The Adders, and a couple Mandrill Kindraas also used conventional infantry, usually in garrisons and Solahma units. Can't remember if the Spirits did or not.
Using Total Warfare rules, I enjoyed using Elementals as well as conventional infantry. Lasers, SRMs, or LRMs for the unaugmented troopers, and sped up with either jump packs or suicidal mechanized options. Transports tended to cost too much BV.
The Elementals can run out of missiles, which means your ranged attack goes away if you're not also bringing AP Gauss.