<also votes "heck no"
There's pretty much nothing out there that stands much of a chance against a JD of equal or even somewhat smaller size in the sub 8" class (provided the JD has reason to be territorial - eg: was in there first +/or is paired up).
Putting Frontosas in is going to = dead Frontosas, if not immediately due to a spat, than later on as they do get pretty large.
A 55 is a nice grow-out size for JDs (IMO).