I do not have african dwarf frogs but  i do know that if the food isn't hard our big than your goldfish's mouth then the goldfishes will probably eat some of it.

Frogs can live in fish tanks, but I really wouldn't go mixing them with goldfish. First of all, I don't think they're compatible, second of all, the goldfish will eat all their food so the frog may starve
Sadly, goldfish don't usually work well with other fish even if the water requirements are the same due to their uhm... crazy eating habits.

I feed my fish three times a day, pellets in the morning (sometimes in the evening) and live foods or vegetables mid-day or evening but only small, small amounts!

The Oscar would eat anything, that would fit into the mouth, and since they grow to be 35cm long, you can imagine that their mouths are pretty big in the end

I don't think you need a special tank at all, there are some frogs that are pretty much living with other fish without problems But a piece of floating wood might be good for them to climb up on However, don't think that I know that much about them, it's just what I think after seeing frogs here and there in the pet shops. They seem to have spent some time there

In South America, I believe in the southern parts of the Amazon I have a lot of research to do about their environment too before I'll setup the tank! Oh, and I want to find some cat fish that are appropriate for the place the Oscars live in!

When I was at the pet shop last time, I couldn't help but buy a little pleco ;> He's beautiful with big, dark spots allover his body He should grow up to be around 30cm too, but it takes a couple of years, of course. I named him Bogdan, but I call him Danny
Needless to say, he's eaten very much of the algae off the plants, my tank is sparkling!
And the salesman was sharing lots of amusing stories about when he kept Oscars, Pearl cichlids and plecos in a 600L tank, very nice to know that they're all compatible!

You have a point there about them frogs... maybe you could feed them some bugs now and then? Like mosquitoes or flies? C:

I asked about this fish if he has any potential to harm the goldfish (just to make sure), and I was assured that he's a very kind and sweet companion to the goldfish

What kind of plecos are yours? Their colors?

Aww, that's cute! Mine has pretty big, dark spots allover him, so yep, probably like the usual plecos do c:
They had a few black plecos with white stripes in the shop, but they were 60€ each! That kind can't breed in aquaria, so they're being caught in the wild. Not zebra plecos, but something very similar, the stripes were different

That's pretty strange... maybe they're not used to that kind of food? Danny has been sucking on his algae wafers without any issues They should start eating it soon!

I always cut veggies into tiny pieces so they comfortably fit into their little mouthes.
Goldfish haven't got teeth like us to bite pieces off. They swallow the whole thing and then in their throat they've got teeth like little bones to "mash and grind" their food.

UUUUPS, I think I forgot you talked about plecos, SORRRRRRRRY, I had the goldies in my head     

LOL, this is so true