Plasmids belong to incompatibility groups as a result of the structure of RNA I and its interaction with the replication primer. See chapter 1 of Maniatis. You can combine (one each): ColE 1 (PBR322/pUC plasmids), P15 (ACYC plasmids), R group plasmids, pSC101 origin plasmids in one cell. <http://www.biowww.net/forum/read.php?f=1&i=5289&t=5274>