Puffers are VERY sensitive to meds. I would also be concerned you would open the fish up to even worse infection if you cut off a piece of his fin. Please post a pic of the fin in question.
I agree, wholeheartedly, with Pufferpunk, a photograph would be very helpful to confirm that this is indeed lymphocystis. There's always the possibility that this is something else entirely, though your description that it looks like cauliflower describes lymphocystis.
Since lymphocystis is very contagious, and, if this is in fact what this is, early action may stop the progress of the infection and also help prevent contagion of the other fish, the snipping off the lump that is on the outer edges of the fin is relatively safe and the fish should do well if provided with optimum conditions of perfect water quality.
If it is lymphocystis, stress is one of the factors that can increase the risk of spreading the illness in the infected fish, and that's why I expressed concern about possibly using sedation if you do opt to trim the lump off.
I've never had to trim or cut away any lesion from a fin, but I have read many articles by our members here, the goldfish gurus, that have used this technique in the past successfully for a variety of issues, and have had no problems with the fish healing up nicely.
If, however, this is some other issue, perhaps a bacterial infection, a different course of action may be more appropriate.
Photographs of the area would be very helpful.
