Unprepaired for rainbow fry help please

[Kabong]

In the process of swapping from a 50 to a 70.
It appears that some rainbow eggs hatched instead of being eaten.
I now have 20 or so rainbow fry. (Unknown species)
The Idea of just letting them die doesn't appeal to me.

They're about 2 to 3 mm Size probably doesn't help since there unknown spies but hopefully it will help for feeding.

Since I'm not prepared I'm don't have everything to usually needed to keep things well.
So opinions would be appreciated.
Feeding options are as follows.
Heb's fry bites
Cyclopes
Reef-Roods (very small size for filter feeders)
Phyto-plankton

Tank options are
Leave them in the 50
The plus side here is lots of water volume
While i don't have a bubble filter, My Ac's intake is low enough in the water that they aren't near it, and the tank is long enough they can stay away from the output.
Down size
getting food to them. (Over feeding shouldn't be much of a worry though the tank still has apple snails hundreds of cone snails and cherry shrimp to clean any wasted food)

The other option is to transfer them into a 3.5 gal
Still no bubble filter so i could either set up a small AC filter with a low down intake or just and air stone an do daily water changes.

Any opinions on what to feed and where to leave them would be greatly appreciated.