I think they should be fine together.

Have you got your fruit fly cultures yet? They take about 7 to 10 days to start producing if they are cultures just made.
If you have picked up cultures already producing, get t least two cultures made every few weeks to keep up with feeding. i always do two in case one fails.

As for feeding them you can put some banana in a small plastic lid or larger bottle cap and place that in to let the banana rot aaway and attract the flies to make it a feeding station. Put them in two locations and that should give the smaller frogs a chance at either the loose flies or the ones at the stations.