There's a slightly hidden limitation in this game's engine. There can be no more than 3 unique palettes for monsters. There is a way around this (only if the crystals are only using 8 colors each), and that's to split the palettes into halves. Set the two problematic crystals to use the two palettes already assigned, and work the other eight colors so all 4 look correct.

If more than 8 colors each are used, you are essentially SOL.

Mould 7 should work, it has one large monster that should be big enough for Culex, one tall small monster, and four small monsters. Put Culex in slot 1 and the crystals in any of the other slots, it should work fine. If this formation will appear on the Veldt leave slot 6 open; you don't have to but Gau needs slot 6 open to return. The original game sucked in this regard, way too many formations prevented him from returning for no good reason.

I missed your second screenshot. 

Hacking the moulds is something we had absolutely no luck with, you'd be better of resizing Culex to fit the mould.

I find it funny how limited FF6 is in this regard compared to FF4, FF4 stores these in with the individual monsters' data; bosses generally get custom moulds while normal enemies use common ones in varying sizes. Of course you wouldn't be able to do this battle in FF4 either because it's limited to three types of monsters (But eight monsters total...another FF6 downgrade).