No, plastic boots won’t be overkill for the climbs you anticipate. I should think they’d be pretty much de rigeur for a climb of Whitney in February. And while heavyweight leather boots would be fine for Rainier in September, you won’t feel silly wearing plastics.
But, you don’t need a hugely expensive expedition-weight plastic boot. Koflach’s Degre boot ($255) would be fine -聴 a flexible, warm plastic boot designed for hiking and general mountaineering. Ideal, in my view, for winter snowshoeing and summer glacier travel. Asolo’s AFS Guide ($350), if you can find a pair, would be another good choice. You might also find some Scarpa Invernos at closeout prices of under $300 聴- that excellent boot is being phased out, I believe, as Scarpa brings in some new models. But the Inverno is best if you know you’ll be hiking almost entirely on snow. They’d be a bit stiff for the relatively dry, even rocky approach hike for Rainier in late season.
Of course, there are leather boots that would be perfectly fine for the trips you suggest. I’d highly recommend you take a look at Scarpa’s Thermo Cerro Torre ($375). This is an insulated leather boot with a sole that’s stiffened a little to better accommodate crampons. Leather, of course, always will have more long-term comfort, and better foot-feel, than plastic. Tecnica’s Altitude Plus, meanwhile, combines leather and synthetic materials for a boot that’s warm, dries fast yet is more comfortable than plastic. Either the Cerro Torre or the Altitude Plus would work very well on the trips you have on the calendar.