I know that the inch per gallon rule isn't really that accurate but I'm curious where you got 24 inches from. Based on my research of the different fish at their adult size I've come up with...
6 Danios @ 1.5"
3 swordtails @ 4"
3 weather loaches @ 9"
1 bristlenose @ 5"
Total = 53"
Tis true the 1 in/gal rule is bullocks. I believe you can stock your tank many different ways amd if maintained properly one can keep a healthy tank. Since you do not have to worry about agressiveness, it will be best to consider adult size and waste production in stocking your tank.
As for measuring...I did not add in the bristlenose. Missed that when referring to your other stocking post. They are more "poopy" than your other fish.
As for weather loaches being 9'', tis true...but I have never seen one ever get that big in captivity. I didn't even know they were supposed to get that big until I looked it up just now. I thought they only reached 6" or so. I have heard of these guys in Canada in the drainage ways. There are news reports of them getting pretty large there
, even though they are not really supposed to be loose in the drainage systems. I think people dump them in there.
Having said all that, the bioload for your tank with what you have selected even once they are all full grown adults will be fairly low. And unlike the other fish you have selected, guppy males are only 1.5-2, some even smaller depending on their genes. Don't count their tales in that measure.
I would not worry too much about being overstocked with adding three guppies. I am guessing that everything you are stocking is not yet full grown yet...which means less waste due to smaller portions of food. They will all grown up and into your 50 gallon tank. As long as you don't allow the mollys to being a fry factory you should be able to maintain your tank with weekly water changes. You will know if you are overstocked once everyone is an adult as you might see your water parameters become unstable with the same maintanence routine. I doubt this would be the case with what you have selected assuming that you would not add anything else and would not keep any molly fry in the same tank.
I would be curious to see how big your loaches do get. Be sure to post pictures of em
.