I'm looking to start using underspins 1/4 oz or lighter on a spinning rod. What length, action, and power do you guys think would be best?

A ML/F or mod fast in 7' should cast those well. Keep in mind there are no industry standards for Rod specs so shop with an open mind.

Id also go with a ml or med with a fast tip. Thesevaresome of my favorite baits, especially when fishing areas with potential forseveral species of fish. I fish mostly 3/16 or 1/4 oz custom pour heads with a fluke or super fluke for trailer. I fish these and a 1/8 oz head on a medium fast spinning rod and use a casting rod in med fast or spinning in mh/fast for anything heavier in weight. I usually go with spinning tackle for longercasts, but sometimes use casting when i need to hit targets more accurately

Keep in mind which model of underspin you are using. I find with the basstrix's stout hook, a M or MH works great. Otherwise, M should do for lighter hooks. 6' - 7' would all be your personal preference