You can plant pot grown trees now, although it's not the best time. Personally I'd wait until the dormant period. You will gain very little by planting trees now - you certainly won't get any crop.
But anyway, best thing would be to plan out your fruit tree area on a piece of paper first, making sure you give them adequate space, can't stress how important this is - have a look at a good website like Keepers or Buckingham Nurseries for guidance. Look at the forms available - if you want a lot of different varieties of apples and pears, you may want to go for cordons, or if you want it pretty, maybe espaliers. The plums are grown as bush forms.
It's really important to keep them well watered. They'll probably need staking too, as they'd tend to be dwarfer types in pots. You will need some support for the grapevine. If you intend to protect the crop from birds, you may want to build a large fruit cage first. Hopefully you'll decide that there is enough preparation to do to keep you occupied until the dormant period!
The other thing I'd say, is that although many are described as self fertile, they always seem to give better crops if you buy two of the same variety.
