You need to be careful of a few things, first of all are the roots, if the roots are where you want to put the pond then it'll be very hard to remove them and also have the change to pierce the pond liner or mishape the pond.
Also you need to be care of things falling in the pond leaves, fruit/berries or flowers which could be poisonous.
In terms of the size, it would be better if it was slightly wider and longer but would suffice, until they are big
Well you need to move onto normal food once you've read the garden peas once. The peas are a laxative so the fish are going to get worse if you feed nothing but peas. Now you've cleared them you need to start feeding them up to strength.
It is done if theres poor water quality, but you could ahve poor water quality if somethings got in which normal tests cant test for
Only treatment you could add if you wanted to was salt and possibily a strong anti internal bacterial treatment.
If the ponds a 175 gallons then you dont ahve to have a filter but i would recommond it and i would allow for more fish than if you didnt have one. If you didnt want one you could plant it out well and create a natural pond with its own ecosystem but i would suggest a small "fountain pump" for areation at least.
In terms of little pumps look at the bladgon mini pond pumps, very good and very cheap (well where i work it is)
Aquarium water treatment will work but are very costly due to the amount of water they treat, thats why ponds ones are better.
P.S. In terms of gravel at the bottom, if you do you will regret it when you come to clean the pond out :D. You can get liner with gravel glued in which would work but its not cheap.