Exhausted fixer is a definite no-no for drains -- the dissolved silver is apparently pretty bad for the bacteria that to most of the work in both septic tanks and municipal sewage treatment facilities. However, there are a couple very viable solutions above for precipitating the dissolved silver before disposing of the remaining solution, which is then harmless (and can be used as fertilizer -- for which I wouldn't recommend selenium toner, since selenium can make harmless plants deadly toxic).
Stop bath is no more harmful than salad dresssing (less so -- the vegetable oil in salad dressing is bad for sewage treatment). Wetting agent is a mild detergent, used in very low concentrations, and as such has far less effect that dish washing soap (either hand wash or machine variety) or laundry detergent.
For developers, it depends. Sodium sulfite isn't something you want a lot of in surface waterways, and some of the phenols in developers can accumulate in surface waters and such, but conversely, most developing agents oxidize pretty rapidly in the environment into simple organic acids similar to tannin -- of which, in many parts of the North Temperate Zone, oak forests add enough to surface streams to make them look like iced tea, without harming stream-dwelling wildlife.
One of the developers I use I could literally drink -- coffee, optional vitamin c, and washing soda (the soda wouldn't taste good, but in small quantities isn't significantly harmful). There is no solvent action, and it doesn't worry me a bit to put it down the drain.