as long as the other fish stay a deal larger than the piranahs i see no problem, piranahs attack in groups and are put off attacking larger fish in groups of 6 or less, although that rtcxtsn will chow down on most of them fish, they grow larger than red tails or tigers, also i find theyre temperment is much worse.
yeah hope so because this 130 gal is way overcrowed. i'm not a specialist in all the speecies you have in there, but some of these fish are territorial. and keeping piranhas, well at least the pygo gender like you have, should only be considered when having at least 4-5 specimens in the same tank.
right now, you are avoiding to have your piranhas eat the others because they're only two. good thing: they won't attack much the others since they don't act naturally. bad thing is they are not comfortable, you have zombie fishes that have nothing close to real life attitudes that come with that fish.
best thing for the piranhas would be to get a larger tank (like your eventual 800 gal) and get few more of the same gender, but then, you'll start to se more territorial issues and you'll loose basically everybody else in the tank...
so my advice is to put the Ps in another tank with more of them, or maybe keep 4 to 6 in your 130 gal when your project is over, and put the other monsters in the 800.