Is the grafted variety that is there now something that you like? Out of curiousity, do you know what they are?

I have a feeling if you chopped out 90% of the trees you would just provoke them to send out a ton of new suckers. What I would do is trade for scion wood http://www.nutgrowing.org/scion.htm or buy some from a place like this http://www.masonvilleorchard.com/scionwood.htm
then I would top work all 9 trunks using a grafting method like this http://www.youtube.com/watch?v=XpgbpbewU3E then you would have fruit in just a couple of years from the new grafts and could still keep the original fruit tree and the roots of the others. With vigorous rootstock they always come back when you cut them down to one stock. If any of the grafts did not take I would then cut that stock out. Do you know what plum they are? The root stock sound like American plums http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0CDgQFjAF&url=http%3A%2F%2Fwww.ag.ndsu.edu%2Ftrees%2Fhandbook%2Fth-3-39.pdf&ei=kbg7VMPvJqf28QH3yYHYCA&usg=AFQjCNG_4yacSh8idfYdggUukUGd_1tMcg

A picture would help, but my take on plum clusters like this is that they will compete and crowd with one another to the extent that you might as well just have a hedge. They're impossible to work or get into and don't produce nice-sized fruit. The rootstock could be seedling plum or something like Marianna or other multi-purpose rootstock. Fowler has a good chart that shows the options- quite an eye opener for me.

If the cluster isn't too impenetrable you might dig out the unwanted sucker trees as thoroughly as possible, and then keep clipping the new ones as they appear. I've had to do that with a prune plum and it's a chore but it works. Of course, when you prune the original to shape it it'll throw off more suckers!

Just my two cents worth. Like I said, pictures would make it easier to comment.

Here is a link that might be useful: Stone fruit rootstock options

I've seen something like this with Stanley on Marianna 2624. The Stanley is such a slower grower that any Marianna suckers are always trying to take over. I laugh when I see literature describe it as prone to mild suckering. That may be somewhat true with vigorous varieties, but with less vigorous varieties or stumps it will sucker profusely. This rootstock needs to be cleared of suckers at least once a month during the growing season.