You should be fine with 1 ADF and 1 Betta in 5 gallons or 2 ADF and 1 Betta in 10 gallons. But I would leave out the Otos since it may over tax your filter. The frogs would need hiding spots but I don't know about pvp pipe. I tihnk as long as they have at least one cave to hide in they should be fine. But you may need more than one cave just incase your Betta wants to claim the cave as his territory. 10 gallons whould give you enough room for 2 or 3 hiding spots.
I've heard horror stories about every type of substrate.
Sand can be passed through if it's swallowed in small amounts while eating. But it does become a problem if the frog eats too much of the sand. I can't find out why a frog would eat enough sand to clog his/her digestive system. So, I'm taking this "death by sand" scenerio with a grain of salt.
Gravel that is small enough to swallow but too big to digest can cause a intestinal blockage and lead to an untimely death. Unfortunately, I haven't found any info on the actual size of said gravel. The people who warn against "Not too big but not too small" gravel need to look at their bag of substrate and give me an actual coarseness rating. Considering how many pictures of ADFs on gravel are online, I'm wondering if this size
is the culprate. But I'm not sure yet. T_T
I've also heard of frogs being trapped under large rocks and drowning when they can't make it to the surface. So I can't go supper big to avoid the "death by gravel" scenerio because they will swim/crawl under a big river rock and become a pancake or get a limb stick.
Therefore, I'm eventually going to have 3 ADF in a 10 gallon together with sand substrate when I have the money. My bettas have had sand substrate for a few months now without dying so I will assume my frogs won't get bored enough to eat sand instead of food. But after the little bit of research I've done for ADFs, it seems like they can find ways to bump themselves off. XD