Say you only need to replace one pushrod. All the other pushrods are steel. Is it ok to replace the one with a chromemoly?

Its ok, but you need to have guideplates with chromeoly pushrods.

I'll just state that pushrods do not fail on their own. Its pretty rare to find only one that is bent in an engine.
The bent pushrod is an indicator of some other valvetrain failure.

------------------
1971 Camaro
383 stroker ~500HP
M21 4spd
12" brakes
16x10" Wheels
Autocross competitive
Engine Designer - The Speed Merchant, San Jose, CA
http://www.speedmerchant.com/

Yeah the valve spring is broken too.