Bichir size depends on species. The largest maxes out at 39", the smallest hit about one foot. The ones you most commonly see are dinosaur eels and are P. senegalus, which hits 12", some rare specimens could reach 15" or so. P. delhezi also stays about 12-14". Synodontis are not actually that big, huge ones are about 7". Three in a 4'x2' tank isn't bad. One would be good, they are an effective bottom feeder and I have seen them with red devils, jack dempseys, jaguars, oscars, etc. The bristlenose don't get big enough to not be an easy target for the cichlids.
The only problem I would see with the bucktooth tetras is actually finding some. I don't think I have ever seen them in an LFS. The giant danios are very common and will definite work.