+1....Talk to Richard at Tampa Bay Saltwater. He will help you out
On 20 lbs, I would say the risk is rather low.
Of course, nothing in this hobby is a guarantee. And "what's the risk" is too complex a question for a short two-line response...
I started my now 2.5-year-old 24g with TBS live rock. To this day I still find new life popping up in the tank ... and love the wonder and surprise of it.
There will be very little ... if any ... die-off on the rock since it ships overnight submerged in water. He ships it submerged in water precisely so you won't have to cure it for purposes of ammonia prevention. Just double check to make sure no big patches of sponge died (not likely to happen, but pick 'em off if there is), then add to your tank.
You will get both good and bad hitchhikers. But it's very unlikely the bad will be 'immediately' damaging to exiting corals and I doubt any will pose a threat to your fish. More-so if you're only getting 20lbs.
The most 'feared' hitchhiker that might show up is mantis shrimp. (I saw none in the 20lbs of rock I got.)
IF you should happen to get one, many people spear them with a skewer if they spot them after rock is in the tank. Preventive option would be to inspect the holes of the rock very closely before putting it your display and, if you suspect there may be mantis hiding in one, pour some sparkling water (e.g. seltzer) in it. Usually causes them to bail right out.
The more likely 'bad' hitchhikers include whelks, limpets, and gorilla crabs.
When the whelks come out at night, just pick off the rock and flush 'em. Many consider keyhole limpets reef safe, but I had one eat some acans. Many people consider gorilla crabs troublesome hitchhikers ... but they failed to live up the fear-hype to me. To this day I still find one occasionally. I used to trap 'em in a shot glass (using scallop as bait) but no longer bother. Never had one harm anything. They act as CUC for a while. But then usually disappear. Not sure if my hermits take them out or they simply starve to death since I feed my tank lightly.
That said, TBS rock is FILLED with amazing life: sponges, little corals, tons of barnacles, tiny pistol shrimp (the size of a peanut if that, hear them but almost never see them, non-symbiotic with gobies), porcelain crabs, etc. - and most of it will die if you do a normal curing process.
If your dead-set on not having hitchhikers, a better bet would to QT it for several weeks, or just get standard 'live rock' from a LFS or someplace like Premium Aquatics (You pay $$$ in shipping for TBS to get the life. If you don't want the life, it'd be cheaper to this route.)