The rule of thumb I use is the surface area of the tank divided by 12 for fish capacity in inches.
ie; a 48"X12" tank has a surface area of 576 sq inches. Divided by 12 it can therefore hold 48 inches worth of combined fish length. (this equals 12 fish at 4 inches each)
alternatively a 48"X18" tank has a surface area of 864 sq inches. Divided by 12 it can hold 72 inches worth of combined fish length. (this equals 18 fish at 4 inches each)
I have always used the anticipated adult size of my fish when determining how many fish to put in so that a tank that is okay now does not become overstocked when then fish mature. Also I don't thinl you would want to make up your 48 inches worth of fish with to 24 inch monsters.
Is the siamese fighting fish male or female? If male, then TBH, he'd be better off in his own tank. If female, still the same but I suppose its OK to have her in there.