The high/mid will be taken care of by the JBL's.

If you have the behringer prokick18000 whatever then I dont think you need HD15's as well, just leave it at that.

You will need two amps, and an active crossover.
1 amp for bass, 1 amp for mod/tops. The active crossover splits the input signal (from your ipod decks or whatnot) into bass output and mid/top output.

the system you suggest would be an expensive way to go really unless you have it all already.
You would be much better off buying second hand proper party kit on here.

Sure you can, amps channels are independant. Put the mid and high signals into different channels and wire to cabs appropriately.

You won't need a JRX125 to keep up with that, a 115 would be adequate. In fact I wouldn't bother with a 15" mid driver as the HDs will reach that, a 12" would giver better midrange definition. Only advantage of the 15 is that it would (perhaps) handle lows better without the HDs. From experience the JBLs on their own need a good 8dB of low pass boost to meet the HF with any conviction for dance music especially.

Sack off the Behringer sub and build 2 HDs. I used to have exactly that stack in my old living room for a while and used it for a house party or 2. A proper management system instead of a cheap analog crossover and as a bonus a good 31 band EQ will yield more than acceptable results. Two HDs off one amp channel and a mid/top off the second would work.

If you need more lows then EQ the HD's, you limit the overall power handling before excusion at these frequencies (probably) but how loud can you get away with for most house parties anyway?

Worry about EQ last of all, it's a nicety not an essential (although some may disagree).