Using random crab apple for root stock could it work?

FB's your man for a more technical answer but yes the rootstock (and local conditions) give the size and vigour of the tree, and the top gives the fruit type.
You are right again that a pip from an apple won't necessarily give you the same fruit. As far as I know you need to graft apple onto apple etc.
If you are not too bothered by the end results I would give it a go as you could teach yourself a useful skill which you may want to use in the future. Good luck.

You should be able to graft apple varieties onto your crab apple root stocks and, as WendyC says, the mature size of your of crab apple tree should provide a guide to the eventual size of your grafted tree, although the inherent vigour of the variety (scion) you graft on will exert an effect. (see FBs threads on variety x rootstock interactions).

I guess that you would either use a whip and tongue or saddle graft, and probably graft them in late Feb/early March 2014, in situ (i.e. don't cut the suckering crab apple root stocks off from the main tree until your grafts have established for at least the first growing season - then, in the winter, cut off the stock with a generous amount of root attached and transplant it to its new position). Alternatively, you could cut off suckering root stocks from the parent crab this winter, with a decent amount of root attached, plant them in another spot and graft them in late Feb early March 2014, or, for a more vigorous result, in late Feb/ early March 2015. I'd probably go form the 'in situ' option, if it's primarily an experiment.

All apple trees grown from seeds/pips will be different from their parents (i.e. will not be the same variety in terms of looks/ taste etc). The differences will be much greater where the pip has resulted from cross pollination between two different varieties, but even where it has resulted from self pollination, there are likely to be small but significant differences in the phenotype (i.e. how it looks , tastes etc). This is because the process of meiosis, which produces the gametes (i.e. pollen and egg) involves several kinds of gene shuffling, including 'crossing- over' (exchange) between the matched pairs of chromosome in the diploid cells producing the gametes. After fertilisation, each gene is once again present in two copies, in the seed - these two copies, one on each of the matched pairs of chromosomes, can either be identical (e.g. AA or aa) or be different ( e.g. Aa). In general, no 'new' genes or new versions of existing genes are going to be produced in a self-pollinated seed. However, if the two copies of the 'example' gene were different in the parent tree (i.e. Aa), then, in the seed resulting from self-pollination, there are three possible options for this gene (Aa, AA and aa). If we assume that copy/version 'A' of the gene is the dominant one (i.e. it gets expressed and affects the plant, whilst version 'a' is suppressed and remains silent), then seeds with the two copies of the gene as AA or Aa, will be identical to the parent with respect to the trait controlled by the gene. This might be something like redness of the skin. However, seeds with the two copies of the gene as 'aa' have a good chance of being different from the parent with respect to this trait, as the version 'a' will, by definition, be the one that is expressed and this may affect the trait differently, i.e. cause a slightly paler skin colour, in this example. There are some very good animated videos of meiosis on the web which make these processes much clearer than i can. They also deal with the situation faced by triploids- well worth finding and watching about twenty times .

The rootstock will be a big factor in the final size but the apple itself will also come into it. A triploid apple ~Bramley will be bigger, in general, then a duploid apple (most other apples) on the same rootstock. And there are variations within apples as well.

Thanks for the answers. I think I understand the variation from parent plant or plants its like if 2 parents are clones the child will not be. So I assume that when varieties are first made by crossing plants they cross hundreds of them and select the best. Also that means all of each variety came from 1 tree?
As for the size I wondered if there is a link that works anything like a bramley will grow to twice the size of the crab apple and most varieties to 1.5 times the height or something like that?

Wouldn't your crab rootstock also produce a lot of suckers?

Thanks again for the answers I am going away for 10 days but will try to see if theres any more when I get back.