Forcing rare enemies negates the point of them being rare, multiple instances of the same floor can be called, but in most cases disabling the town warp is necessary, in this case though not needed since boss floors, for an example of multiple floor reuse see pw4 or cmode.
Fixed box drops as well as advanced use of the bb opcode for bpd can award items, changing boss drops requires sever code.
Multiple bosses at the same time is a stupid idea unless you like to crash, spawning them in waves can work as long as the delay waits out animations, again unless you like crashing clients, also don't wave spawn multi stage bosses, ahem falz, or sequence animation bosses ahem de ro lie.