I assume the length is 4 feet (120 cm) for a 75g (one of my tanks is a 70g), but please confirm. Silver Dollars would basically fill this space (upper levels), a group of six or seven. They will usually eat plants, so this should be kept in mind if live plants are intended. There are several species under the common name, but all share these traits.
Aside from that aspect, I would not recommend pictus catfish (Pimelodus pictus). First, they are very active, and this can be quite disconcerting to quiet and sedate fish, such as Silver Dollars, especially at night. Second, being a shoaling species, it should have minimum five in the group. As it matures (it attains five inches) it will likely eat smaller fish. It is also a very greedy feeder, devouring any food it finds, and feeding other fish (especially more sedate feeeders) is difficult. Pictus are fine with one good feeding a week. Third, it requires a stronger water current than the other fish mentioned.
If Silver Dollars are the focus, I would not include Buenos Aires Tetras (Hyphessobrycon anisitsi). This fish will attain 3 inches (some sources say 3.5 inches) and need a group of at least eight. They are prone to fin nip sedate fish, which is why they are not suitable with SD's.
The Bristlenose is fine with SD, or similar fish. Substrate fish like a decent sized group of Corydoras catfish wold work.
Byron.
