Plum, Blue Tit, Pixy Rootstock Questions.

Pixy would be OK in a pot.
It would also be adequate in open ground if the soil is at least average fertility and has adequate rainfall/moisture retention.

If you get them at a young enough age (so the roots will fit in the pot without breaking), it is possible to grow almost any rootstock in a pot. Just think of it as a Bonsai tree, or a root-restricted form (which is what is used to control the spread of vigorous fig roots).
When a root hits something hard (such as the side of the pot or a big rock in the soil) the root sends a signal for the tree to slow down growth, to give the roots more time to grow around the obstacle and into nutritious soil. So if the roots are regularly hitting the side of the pot, the tree will be dwarfed.
Summer pruning (July) also has a dwarfing effect, as does less feeding and less watering.

I am confident that I could grow a very vigorous fruit tree rootstock in a pot, by controlling the moisture and nutrients. However, a vigorous rootstock would need to be planted at a very young age (ideally only a 1yr maiden) otherwise the root spread would be too much; a vigorous rootstock can easily spread 2ft in each direction for each year of age of the tree (sometimes vigorous roots can grow 1ft in just several weeks!), so they very soon get too big to squeeze into a pot unless the pot restricted them when they were as big as a pencil and roots only a few inches long.

Thanks for the response FB!

Just one final question, I appreciate it might be a annoying, but could it be put in a pot for a few years then moved into open ground? Would I be wrong in presuming if I pruned the roots it would stimulate them into new growth if they were moved into open ground?

Cheers

I prefer not to cause any more damage to roots than necessary when transplanting. This is especially true for dwarf or semi-dwarf, which have relatively slow root growth and may struggle to repair the damage.
Roots and their suitability for the soil/climate/growing-conditions where they will be grown are the most important part of the tree, but most people only concern themselves with what's grafted on top.

Apart from untangling the matted roots before planting a container-grown tree, I don't like wounding the roots because many of today's rootstocks are becoming increasingly prone to diseases - sometimes brought with them, or in microscopic traces of the old orchard soil smeared on the roots from the nursery.

It's rather like the commonly-grown "old favourite" fruit varieties (e.g. Cox apple, Victoria plum, certain "old" strawberry or currant types etc) tending to be prone to disease because they've been grown in the same nursery sites for decades; the same old rootstocks have been grown in the same old nursery soil for a long time and now the soil is badly contaminated with diseases which have evolved over many years to be very good at attacking them - and soil-borne diseases at the nurseriers are not easy to eradicate; you can't spray fungicide several inches below ground!