To be honest, the idea that more filtration means more fish is outdated. The amount of filtration is not important. What is important is that the tank can remove all the ammonia made by the fish. And it can be either bacteria or plants or both that remove this ammonia.
Adding more filtration does not mean you can add more fish, except that possibly you can add a few more with an external as these effectively increase the volume of water in the tank. The concept behind the community creator allowing more fish with more filtration was current when the CC was created several years ago, but modern thinking is that filtration should not affect stocking provided the filter can remove all the ammonia.
It is quite possible to run a tank without a filter provided it is heavily stocked with fast growing plants which will remove ammonia as fast as the fish can excrete it. And of course there will be micro-organism of all sorts, including ammonia and nitrite eaters, living in the bioilm on every surface in the tank, and in the substrate.
There are so many other factors to take into account when stocking a tank.
Did you know that if you have 12 neons they won't make double the bioload of 6. This is because the 'happier' a fish is, the less it affects the bioload, and shoaling fish are 'happier' the more they are. The metabolism of stressed fish is higher than that of 'happy' fish.
I would always use the 'standard internal' figure for stocking, with maybe a bit more for externals but not as much as the CC says.
So yes, get the guppies
The other old belief about filters is that there is no such thing as over filtration. There is such a thing. We have to take into account the behaviour of the fish and the nature of the water they evolved in. For example, angelfish. These fish don't swim as such, they cruise along. Put them in a high water flow and they would be very stressed. They need slow moving water. But other fish come from fast moving streams, and they need a high water flow. These fish would be stressed in the slow water flow needed by angelfish.
All the fish you list are slow water fish.
But looking at your post, I would increase your water changes to 50% weekly.
What is the nitrate level in your tap water? Tank nitrate should not exceed 20 ppm, though with the ampunt in tap water in some areas, that is quite tricky to achieve.