A small amount of coco fiber wont harm the frog. Its the safest substrate if accidentally ingested. Of course you want to minimize that as much as possible.

It is best to measure the temp and humidity levels near the substrate. About 1" to 1.5" above the substrate. Abient air temp and humidity are what you want to go by as well. It is good to have a temp gradient in th enclosure though. Most of mine burrow on the warm side even if the average ambient air temp is 84°. They pick their favorite spots and dig in. In a 10 gallon you'll want to get an average reading of the temp and humidity in the enclosure by haveing the gauges beside each other in the center of the back wall of the tank 1" to 1.5" above the substrate. This provides a good average reading.

Do not place a UTH on the bottom. Since they burrow to cool off the frog may burrow over the UTH and over heat itself because it will try to burrow deeper not understanding why it cannot cool itself. You can actually wrap a towel around the bottom of the tank to help insulate. I'd perform the soaks and keep a close eye on him. Probably go out and buy some Fluker's Repta-Boost just incase. Don't force feed him too much too often. Keep stress low.