If you use a 25 w greenback by itself with the master dimed and the amp in variac mode, I am not sure that would be the safest way to go. It might be ok for a while. Seems like you would want to be safer with the speaker. But, probably best to let John chime in.
You did not mention the speaker your using. I have heard you should use 2 times the wattage on speakers over the amp. 25 watt amp would be safe to dime on a 50 + watt speaker/s. If you are not diming the amp when it is in variac mode, nice room volume, you should be ok with a 25 watt speaker. I have a Heritage. I believe it is a 30 watt speaker and I have been playing my Carvin X100B at 25 watts for about 4 years and have no problems. Yes, a Carvin.
Use a creamback or something... You can even run the amp at 50w and still be fine. Unless you turn it all the way up... That will put out more than 50w i think