Don't prune them now, you have plum and cherry and being stone fruit they can be susceptable to silver leaf which will kill them. Late summer is I think best.
Pots and compost:
Really as big as reasonable for pot size, I have potatoes in 18" dia by 18" deep pots for fruit trees I would be aiming for something similar.
I have for next years potatoes some tubs that are 20" diameter and 15" deep, I would think that those would be better for fruit trees. The container should have a couple of drainage holes in them - 2 or 3 1cm holes in the base.
The compost - if it was general purpose compost you need something more solid/heavy. General purpose compost tends to be something that has nutrients added which are soon used up or washed out.
Many will suggest John Innes No3. I bought some once tried it and found it fairly poor, rock hard and generally not what I had expected to find. I wanted it for a lime and have ended up with a mix of compost, some of the JI No3 and well rotted manure and some sand.
They are a tree so being outside should not be a problem, may be necessary they are triggered by light and specifically lengthening hours of daylight.
Biggest risk is them drying out in the summer, so a layer of mulch to protect against this would useful, the drainage holes should allow excess to escape.
Will they fruit (thinking apples here), difficult to say, usually the dwarfing varieties fruit early on in their lives. So in a good sized container and with good soil I could expect something fairly early as in year 2 or 3. But expect just a small crop at any time and in the first year just 1 maybe 2. You want 1 to try it out.