Keeping tadpoles? Is it really that simple? How many can i keep?

well... its a pond, dug into the ground, then the plastic pond fitted in. then cememented round the sides. it has no filtration system, and only a clump of java moss as a plant, but i have loads of large stones in it that have algae and plants that i am unsure about growing on them. the bottom is filled with smooth gravel. it is about 1.5m long, and about 80-90cm wide, and about 30-40cm deep. as you can see, this is just something i picked up at a garden pond centre. i just saw it lying there, it was only �20. i just though "hey... thats quite big for its price! iv got loads of garden space so i could use it for minnows and tadpoles!" as this is way to small for goldfish isnt it? i tend to give my goldfish at least 100 gallons of water each so they have no problems growing. anyway, back to the question, how many tadpoles can fit in this? i have no idea how many gallons it is and the measurements were just rough estimate. id say its around maybe 40 gallons? i could be totally wrong, it could be more. and yes, i will let the frogs leave and come back as they please. how many minnows should i get? i am very fond of tadpoles and minnows. maybe 10-20? the minnows i will get are called fathead minnows, they come in their thousands along the river next to my house. so taking a few wont do much damage. and before you ask, i have had this pond built in and treated water has been sitting in it for about 4 months now, iv been waiting for frogs and minnows to come. they should come in about a month. so there will be no water problems. also for the matter, is there any type of catfish/loach or algae eater i could get for my pond? i am very fond of them. I live in Scotland, so you can get a grasp of the temp. so no tropicals please.