one thing that I think matters more than length is how thick they are I've recently had to turn down a 5" retro bichir because it was skinny enough that my larger bichirs(12-13") might think it was a snack however my smallest bichir is barely 5" (albino sengalus) but he is pretty thick so safe. I also have 3 ropefish but they are all thick enough not to worry about however I did have to grow one out before it went into the tank. If you do get the delhezi you could grow it out for maybe a month at most using shredded fish daily and your delhezi will easily be big enough to go with your sengalus
anytime. Also next time you post about bichirs you should post directly in the Polypterus sub forum it is incredibly active with experts on these fish. I only recently started checking here or I wouldn't of seen your post. Also good luck with your new Del, I've got 2 a 10" wild caught and a 9" captive bred that I had to grow out to go in the main tank. While growing him out I had an albino sen growing out too and the del grew twice as fast