Adventures In Eberron
Under the normal rules, only the new grafts are incompatible with each other… which doesn’t make sense because you could already make a monster with the old grafts. I will assume that the “individuals who have attempted to bypass these biological limits abound, but such monstrosities … rarely live long” (MoE126) is because of the new graft hp loss. I therefore recommend treating the single graft type limitation as fluff.
Characters only gain the passive benefits for one graft type up to a limit of 5 instances.
You may not replace the same body part more than once. (IE you can’t have 3 Left Legs)