if it can fit into their mouths then they are big enough.
I don't know anything about BSF but I do know Tilapia will eat just about anything but take time to adjust to it. Everyone says they love duckweed but I find they prefer fish food. If I put them both in the tank they eat around the duckweed until the food is gone but will eventually eat it.
Yes. That's my experience too. I just switched them from fingerling food to the larger variety and they've been very slow to take to it. My last batch of fish would never eat the BSF.