In order to assign a "negative LA" and keep it balanced, you also need to determine what effect negative LA actually has for the purpose of your calculations.
Letting you run with more HD than your ECL is bad for balance, because that means epic feats early - and can screw up skill point prereqs.
Subtracting actual RHD makes the feats awkward - now you need to either decide which to drop or which are so essential to the creature that they should now be bonus feats.
Alternatively - and this is what I'd favor - would be that ECL=RHD, and the negative LA gives you however many levels of RHD gestalted with class levels. However, then you need to decide what, if any, restrictions go on those gestalted class levels.
However, this should probably be done in its own thread, as Inevitability isn't doing it here.