Backyard Toad Pond

Hi all,
I am looking to create a terrarium in the back yard for my 3 fire belly toads (I may get more when complete). It is a large half barrel that I used before for a fish pool. It holds about 50 gallons when fully filled and is around 2ft tall with a base of about a 2ft diameter. My current plan is to use expanding foam (waterproof) to create a land area and waterfall. It will have many safe live plants as well.

My 3 frogs currently life in a 15 gallon glass tank with 50% of it being water around 3 inches deep. I hope that making a larger outdoor area where that can get some more sun will help them out (not that they are doing badly right now).

A few things I wanted to know:
1. I live in San Francisco and the temperature goes down to around 50 degrees and maxes at around 85. I plan to have a heat lamp on the frogs 24/7 and if it still gets too cold I will move them into my current indoor cage.
2. I plan on feeding them some small fish. Which varieties if any are good for them? (I feed them crickets, roaches, and mealworms as well)
3. Since I do not have a real waterfiltration system I am creating a drainage system in the barrel to easily drain and refill the water from the hose (I will treat it of course). I was wondering if I should purchase some sort of bottom feeder fish / snail / something to keep the water cleaner and if so what the creature should be.
4. I was wondering if I could keep medium sized fish with the frogs. Ones that would just be to big for them to eat. Would this be safe? If so what are some good species?

Thanks for reading! I need all the help I can get!

I think the fish will eat the tadpoles/toad eggs and of course the frogs if they escaped would only arm animal rights nuts with more ammo to ban exotic pets so escape proof is key.