Katya,
The various Erlkonigin clones come in two diameters---20.5 and 20.7mm, but that seems to be merely a difference in the thickness of the tank section. All the bases, O-rings, and caps are apparently identical in size and interchangeable. Mine are all 20.7mm, and the Mini kits fit all eight of them perfectly (same threading on the chimneys and bases, and the shorter chimneys/
tanks are the right length to seal exactly as they should).
I can attest that the FT Mini kits fit both the $10 Erl and the various $5 Erls in the angelcigs sale. Those ship from the U.S., so they arrive really fast, but shipping is ten bucks, which is why I bought five of the five-buck ones. The two cheapies I've put into service so far work beautifully, however. No difference in vape quality that I can tell between the $5 clones and the two $10 ones I bought a month ago or my first $14 Erl clone from FT purchased in November. In fact, the draw on the cheapest ones is "cleaner" (meaning airier but less noisy) than the draw on my first Erl. Even though I don't direct lung inhale, I like an airy draw. Go figure. The Erls don't have as ultra-smooth a draw as my Nautilus BVCs, but they're easily good enough to keep me happy. Anyway, the draw is infinitely adjustable on Erls, so if you want just a smidgen more airiness than with a Kayfun, you can get that from an Erl with ease. Tight, medium, or very airy---whatever you like.
Below is a link to the full-size Erl clone tanks in the angelcigs clearance, and then another link to the Mini conversion kits from
fasttech:
Erlkonigin clone tanks on clearance at angelcigs
Replacement Chimney and Tank for Mini EK RTA at fasttech