Exposing fish to high levels of ammonia and/or nitrite is not good for the fish. If you stick a whole heap of fish in an uncycled tank and wait, that's what you'd be doing. But if you put a few fish in a tank, monitor the levels and change the water frequently so that the fish are not exposed to high levels, then that is a very different thing.
Although I constantly read about doing a fishless cycle, here you cannot buy the type of ammonia needed. I rang several aquarium shops and none of them did fishless cycling. They told me that they just seeded their tanks and away they went with fish in.
You have the advantage of having filter material from one tank with one fish. You are only initially building your new filter up to deal with one more fish. If you use your existing filter as well it should take care of any ammonia. Your 2 fish will not be exposed to high levels of ammonia - I doubt you'll even see a reading. If you add a fish at a time, with a couple of weeks in between, you shouldn't even see a spike. Your filter will adjust gradually to the increase in stock.