Synodontis are fantastic, clowns are thought by many to be inappropriate. They get too large, the pH of an African tank is not ideal for them, and they are not Tanganyikan. I have both Lucipinnis (group of 6) and S.Multipunctatus (group of 5) and highly recommend either.
If it's what we've discussed before, I don't think that syno cats are a good choice; your gobies and shellies will scavange quite a bit, and if you need further cleanup you are feeding too much. Synos will disturb the breeding of all your species. I keep a group of S. multipunctatus but they've got a dedicated 55 gallon tank now. My calvus never bred and my julies had zero fry survival when they all were in the same tank.
If you want an algae eater, I recommend a bristlenose pleco (Ancistrus spp.).
My Tangs do alright with a group of synos in the aquarium. I've bred every species in the aquarium and they all seem to get on. I do have a very brightly lit tank during the day and crepuscular lighting in the morning and evening before the lights go out so this may have something to do with continued peace. I don't, however, consider my synos to be scavengers. I treat them like any other fish. I feed them at dusk and dawn with the night lighting on and watch to make sure they're getting along. No aquarium needs a scavenger, so if you don't have room and your stocking options don't allow it then don't worry about it. If you find that you'd still like to keep these catfish they need to be in a group and they have their own care requirements.