I wouldn't suggest a frog for that tank. It is a bit too tall for them, since they go up to the top for air every once in a while.
I'd also agree that a frog in tank that tall is a bad idea.
If you're hoping to get them to breed , I'd agree that 10 shirmp is a better way to go, unless you can accuratly sex them. They are a bit hard to find in stores and tend to be expensive- ($7 for a RCS??) However there are a lot of people on here that sell them for about a buck, I buy from Ronw007 , I'll actually be making a pickup this weekendBob's Tropical Plants sells them for $1.50 each, as well as some plants, all on the cheap.
I ordered my cherry shrimp from msjinkzd. Shipping only cost me $8.