My BBGs actually do eat fine flake food now (a mix of spirulina, and tetra products like tetramin and tetra pro), but their preferred diet is bloodworm (I use frozen, they look hilarious sucking one down like it's a noodle). They also get brineshrimp regularly.
I actually have a fair fast flowing current in that tank, with a canister filter, and they're just fine so I wouldn't worry overly about them being sucked up. They stick to the tops of the plantpots I have in there for them, and love goading my stream loaches over territory.
Mollies would love a bit of vegetable/fruit in their diet, yes, so go for the cucumber idea it will go down a treat
I've found my mollies prefer me to cut the cucumber up into smaller squares rather than slices which they have a nibble on. I also feed them spirulina flake regularly (along with other flake products) to give them some variety.
Your BBGs will grow no larger than 1-1.5" I believe.
You may find the Gobioid Research Institute helpful: http://www.gobiidae.com