I'd be very, very cautious on 'instant' cycling.
In the tank pictured below (a 125 gallon, 6 foot aquarium) I used a decent number of live plants (many fast growing) and in addition used Seachem's Stability.
It could not handle the initial fish load of 10 Harlequin Rasbora and 10 Diamond Tetra. 20 fish sounds like a lot, but keep in mind they were juveniles and even full grown would be ~20% stocked at most.
The levels never got out of control, but were measurable (~0.5 ppm Ammonia and Nitrite at various times).
So the moral of my story I guess is that yes those products, and plants, can greatly speed up the process and make it less severe I wouldn't call it a silver bullet unless you have extremely small loads. I think I would have been okay with just the Harlequin Rasbora. In small tanks though, I don't think you could have anything that schools and keep low stocking.
I know others have had better results, but that's what I ended up with.
I missed that you had another tank already set up. You can use its media to greatly speed up the cycling process as mentioned by others.
This is a picture from Day 1. It's very much filled out now =)