invaderwithkey,
I have significant experience with Polypterus endlicheri congicus and you're likely correct in stating that they are the largest scientifically-recorded bichir at nearly 39 inches. It does take them MANY years to reach anything approaching that size and their growth rate drops off dramatically after they've reached 18 inches or so. Their care is, indeed, the same as for any other bichir with adjustments being made for their size, of course. The aquarium they are housed in should be at least as wide as the body of the fish itself. Does that mean if you get one you need to have a 39 inch wide tank? No...but you should definitely keep their eventual size in mind when considering one. Smaller P. endlicheri congicus can be kept in appropriately smaller tanks...I've even used 10 gallon tanks as grow-out tanks for P. endlicheri congicus that are less than 6 inches long...graduating up to 20 gallon Long tanks...then 30 gallon breeder tanks...etc. as they grow. I've only got a couple of P. endlicheri congicus at this time (both around 12 inches) and they're housed in a 100 gallon tank with other bichirs of similar size and some larger Ctenopoma acutirostre and Ctenopoma kingsleyae. I do not recommend housing bichirs with large, aggressive fish like cichlids though some people have done it successfully. Bichirs are definitely predatory but, in general, are not particularly aggressive and often will not defend themselves vigorously against other fish.
Hope this helps a bit,
-Joe