Cloning tanks is a very common thing with folks running multiple tanks, especially for folks who breed & sell, as stocking can vary. Let's say for ease of math you have a 10 gallon tank with 10 guppys. The filter media is capable of sustaining 10 guppys biologically, or half will work with 5 guppys. If you take half the media, and 5 guppys, set up another 10 gallon tank with that you can be reasonably assured you'll have no cycling issues.
A mature colony of nitrifying bacteria is capable of doubling in 18-20 hours. As bacteria go this is rather slow, for ease of math, water changes & my life schedule I like to ballpark it at doubling every 24 hours. This being the case, you can generally take 1/3 of the mature media out of a tank, and add it to a new tank that contains 1/3 the stocking of the donor tank, and expect no issues. For safety's sake it's a good idea to fast the donor tank for a day before pulling media, and feed both tanks lightly for the next week, less food means less waste produced. This helps to eliminate the risk of any spikes. Removing only 1/3 rather than half also leaves a bit of margin for error. By all means test the water the first few times doing this, until you feel pretty confident on how things work out.
Rarely are tanks stocked like our 10 gallon example, much of it is guesstimating. You'll have some smaller species with larger species, so there is a bit of guess work involved. The fasting & light feeding helps to accommodate for this, the testing assures you're headed in the right direction.
Many times I'll get a lot of fish in, mostly angels, 200 to maybe 500 at a time. I flip back & forth between breeding angels & buying overstock from other breeders, 30 tanks & 1k gallons to play with in the fishroom allows me to do this.
I'll often take a tank with 10 angels, and add 100+, with nothing more than a large water change every other day for a week. Knowing the nitrifying bacteria will double every 24 hours, the tank with 10 angels can handle 20 by the 2nd day, 40 by the 3rd day, 80 by the 4th day, by the 5th day it's a done deal. On day 2 it's a 75% water change with a triple dose of Prime, same with day 4, day 6 gets a 50% with a triple dose of Prime & everyone is happy. Prime will tie up ammonia for 24-48 hours, into a form that doesn't harm the fish, but is used the same as ammonia by the nitrifying bacteria. It also does the same to nitrite.
Remember that a mature colony of nitrifying bacteria doesn't mean your tank that just cycled a couple weeks back. Most of the large sponge filters I run have been in service for years, good sponge isn't cheap but it's worth it. Nitrifying bacteria also have been shown to go dormant in a previously heavily stocked tank that is currently stocked more lightly, and rebound to heavy stocking considerably quicker.
So to start with plain cloning, 1/3 of the media, 1/3 of the stocking, feed lightly, test & be ready to do water changes as needed. Beyond that feel free to push the limits, provided you do have a good backup system planned out.