hydnophoras are pretty relentless...i have a rigida thats been growing out and has taken out acros, montis, and anything else i place within 1-2'' of it...
to aswer the question, i wouldnt put anything too close to the frogspawn...they send out sweepers that get pretty long and things start to get messy...there are a few exeptions ive found...ive had a hammer and a F/S live in such close quarters they touched when fully expanded...niether seemed bothered by it..i placed the hammer by a torch and they are currently growing next to each other...i tried it with a F/S head and the torch made short work of it...