Yes, the twin flash is pretty much a necessity with the MP-E 65mm. I use it mostly with that, occasionally with the 100L Macro. For the latter, you'll need the Macrolite Adapter 67C. The MT-24EX clips directly onto the MP-E 65mm. Note that a filter on the front of the lens prevents mounting the twin lite. Personally, I use filters in all my lenses, but one is very useful on the MP-E 65mm - the working distance is so short I'm always pushing the front of the lens against a bush, etc. If you want to use a filter with the MP-E 65mm and mount the MT-24EX, get the Macrolite Adapter 58C and mount that in front of the filter.
+1 on the StoFens, I have them.
Been thinking about the RRS brackets - very nice, although the setup costs more than the MT-24EX itself!