It really depends on who you ask, what specific species you are keeping, and how much your filtration can handle. A general way of stocking would be groups of individuals. 4 groups of about 5 individuals would be a good place to start. You can keep exactly 0 clown loaches in a 75 unless you have a very large aquarium ready for them once they outgrow your tank. I've never personally kept them but that is because I seen individuals a foot long and since they are schooling fish, they are not suited for a 75 gallon. As for synodontis catfish, I have no idea.