It's suprising what they'll eat, even though he's a lazy git and very well fed, my del went through 8 danios in two weeks... Here's hoping the black widow tetras are too big for him...
I have a fully grown female platy in with my two Poly's she is fine, but as stated above, she is FAT lol and they dont even look at her as a meal, the same as the fully grown tiger barb too.
when they get bigger I will have to move them, but they are fine for now.
As for Danios, that was just a silly mix, my apollogies, I dont mean to offend. unless they were fully grown giant danios?
if you want a good tank mate, go for a large synodontis, or talking cat fish (platydoras costatus) even large Clownloachs.
Just keep in mind that you are better off with LARGE peiceful tank mates.
Plecs are a good choice, many people have said use a Bristlenose as they dont tend to latch onto the poly's and suck their slime coat, I had a BN in with mine for months, but I also had a L001 in another tank, and it became TOO big for that tank, so I swapped the BN and the L001, and I've had no problems at all.