The inch per gallon rule isn't a good one. That's like saying you could put two 6'(144") fish in there with a couple guppies. You really should look up one of those tank volume calculators. That will tell you how much fish you can keep. The reality is that the clown loaches will get to 1' in that tank which will be 5'(60"). The stocking you have sounds good, just remember to keep 2-3 female guppies per male. Good luck!
But Clown loaches are very slow growers and it might take 4-5 years before they even get 6" and maybe as long as 8-10 years before they get a foot long, if they even get there so in the mean time you could certainly up the cardinal/neon total to 30-40. As I said earlier, large schools of cardinal tetras in a well planted tank look very nice.