I'm adding a dormer to my house and the roughing for my bowl should be about 12 1/2 off the studded wall, but the problem is that it lie within 1 inch of a beam downstairs. My question is, is there a bowl that will take a 10 inch or less rough.

Yes, but there are also off set toilet flanges.

Yes, there is. It is called 10" rough and it is used in situations like yours. However, you will have bad selection of toilets designs in this configuration. However, TOTO makes special mounting collars that go from 10" to 15". If used, you can use regular TOTO toilet.

You can proceed as Harold suggests with installing off-set closet ring.