Are all hybrid fruits acceptable? (grapefruit? orange?) How, given the prohibition of crossing seeds?
Yes it is permitted to eat he grapefruit etc. even though they do originally come from grafting, which might have been forbidden to do, however the fruits that we are eating are not from the actual grafted trees, but from replanted produce of these trees. The replanted produce of branches of these trees that were replated are not forbidden to eat. Additionally they most probably were planted by a non Jew, and therefore not an issue, therefore they are permitted to eat without a problem..
Rambam HIlchos KIlayim 1-3, 7, Y:D 295-7.