The problem with some plecos they develop a taste for slimecoat and with their rasping teeth they can do serious damage to bichirs, If you want a pleco go with a bristlenose they seem to not bother bichirs.
As for tankmates this can vary with the species of bichir, upper jaws species IE> senegals, retropinnis, aand palmas complex have much smaller mouths then the lower jaw species IE. lapradei,endlicheri, so many tankmates can be different basicaly you want tankmates that are fairly non agressive,and big enouh not to fit into a bichirs mouth