Look at the shape of the tank and compare it to the shape of the bichir. Its a good tank for angel fish, not for bichirs.
You could keep a senegal, palmas, or delhezi in a 4'x18"x18" for life, they all max out at around 12-14", and usually grow slowly after around 10", could probably keep several of any of those species in a tank that size, just make sure it has good filtration and regular water changed because of all the meaty food.
As for the dat, you'll need something bigger, they get to 18"+ eventually, I've had mine 2.5yrs and its gone from 2" to 8", currently in a 4'x20"x20", but I'm shopping for something bigger. much, much bigger.... 8)
*edit* Oh, and as for the floor, try get under the house to see what the piles/joists are like. Design the stand to spread the weight evenly and you'll be fine. I had a 550L tank on a wooden floor for 2 years, no problems.