Hello
My good friend Brandy raises her rudis babies in groups whereas I've had more success with my montane babies when raised separately. That said, I believe the rudis can be raised in small groups unlike other ovoviviparous species like the werneri and merumontanus for example.
Live bearer babies are also notoriously hard to raise in captivity and success rates are still very low. They seem to do well until about 6 to 8 weeks of age and quickly go downhill from there. If you have two weak ones already I would separate them from the others to give them a chance at the food. Their caging need not be fancy. Put each baby in their own small Kritter Keeper or Rubbermaid, add a thin stick or two and toss a plant cutting in the bottom There are some excellent, inexpensive caging pics on this thread already. Steal ideas from those!
As for the aphids... I guess you could try? I did an ever so brief search on them and they aren't coming up as being toxic, but check the species native to your area first. I would think aphids would be too small and immotile for rudis babies to be interested in them?!?!?
Are you dusting the FF's and crickets? What's your temperatures and humidity like?
Good luck. Keep me posted.
Trace