to comment on questions/responses so far:
for a 55g - no lungfishes (commonly) available would work in a tank that size for life. you could use it as a grow-out tank, but it would need to be upgraded. a gilled African lungfish (Protopterus amphibius) could work in a 55g, but those haven't been imported to the US in several years.
for a 100g - you could make it work with the African species for a long time, possibly life depending on the individual. of course with African species, no tankmates.
you could make it work with some bichirs and an SAL, again, depends on the growth of the SAL and your filtration.
SAL and AUL are the community species...so consider those for a large community tank if you are interested. AUL are more expensive but tend to be a bit more docile (and their fins can't be picked off as easily as the SAL) so they may be the better choice for a large predator community tank.
good luck--
--solomon