Yes, it is safe. In fact, you could release games for those platforms right now, using the “Xbox One,” “PlayStation 4,” and “Custom” platforms already present in FMOD Studio.
As far as FMOD’s inner workings are concerned, the only difference between one platform and another is the compression and encoding formats that they support. The list of formats supported by PlayStation 4 in FMOD Studio is identical to that supported by PlayStation 5, so those platforms are identical in FMOD Studio. The list of formats supported by Xbox One is almost identical to the list supported by Xbox Series X|S; the only difference is the addition of the Opus format, for which you must instead use the “Custom” platform.
We do plan to add these next generation platforms to the supported platform list in an upcoming version of FMOD Studio, but you can already design and build content for those platforms.