It should fixed in 17.4. FFA multiplier ignores any thing over 10 million HP.
Re: Random multiple race per planet. That's a neat idea. I think it will likely introduce high performance penalty in order to build that since I'll need to check the enemy buildings location, which race I should replace and etc.
1 race per planet is already running its custom autoplace function, which is somewhat slower than other options which depend on basic game autoplace.
I'll add it to the backlog. I will think about it when time permits.