Several sites way overexaggerate the size that senegals get to because they're either using max size information from fishbase (and the max size of 20" has NEVER EVER been seen by captive keepers - I'm not sure that recorded size wasn't possibly a mistake. The reference fishbase cites is WAY out of date.), or they're mixing them up with other species.
It is generally accepted that senegal bichirs almost never get over 12", and rarely to about 14", and it takes a very long time for them to grow over 8-10".
A 40g IMO is fine, at least for many years. It's very common to keep them in 55g tanks, and those are shorter front to back.
You can keep several together (given you keep water quality good). Territorial or not, they don't seriously fight with each other. In fact even in a large tank the majority of the time they are laying on top of each other.
As far as tank mates, stick with fish that can not fit in their mouth and are not aggressive.