Yes—mixing different brands of manual strobes is generally fine. If you’re using the flashes off-camera in manual mode, different makes and models can work together without issue.

The main things to watch are:

• color consistency: most flash units are close to daylight color temperature, so they usually match well enough.
• trigger compatibility: make sure your radio triggers can reliably fire the new strobes.

The bigger color problem is usually not mixing flash brands, but mixing flash with tungsten/household lighting. Flash is typically daylight-balanced, while tungsten light is much warmer. If both light sources are in the scene, you may see unwanted yellow or blue color casts unless you gel the flashes or control the ambient light.

So for manual off-camera use, mixing brands is usually okay as long as the flashes trigger properly and their light color is reasonably similar.