Rumor has it Black Ghost Knives can get 18", some even say 24"... I have one that is 7+ years old that is 14"... I had one for 5 years that stopped growing at 12"...
So while a very very large specimen in the wild may get beyond this size, I wouldn't expect one to get more than 14" in an aquarium... They will stay quite suitable for a 150 gal tank...
Black Ghost Knives use "radar" to navigate, as they are blind. You will not see this radar in any way and they do not "zap" anything. I agree a tank with nothing but a single BGK would be quite boring... but adding one to a Cichlid tank just adds to the diversity of the tank.
Feel free to breed guppies as treats. Mollies are just as easy to breed but get a bit bigger. Using such things as treats or snacks is fine, but you will want to rely on a quality pellet food for a well balanced diet.
I think the Cichlids you are considering (4~10") are in the perfect size range for your 150 gal (5' x 2' x 2') tank. If you went with anything larger (12~14" fish) I wouldn't advise anything more than a pair and a few dithers / target fish. I think something like a PBass or other fish in this size range are a poor choice for this tank.
I think you are making the right choice for long term success...
You can keep a group of Silver Dollars in this tank with the fish you are considering. I personally do not like using them as they grow fairly large and need to be kept in a group, which takes up space that could otherwise be used for more interesting fish. But if you like them... who cares what I think
Another non Cichlid tankmate you may consider are Clown Loaches. They are very interesting & amusing bottom feeders. In my experience medium sized Cichlids will leave them alone provided the Cichlids are not breeding and the Loaches are not tryign to eat the babies. I don't think breeding Firemouthes would cause an issue with them...