Cut the rock in half to give them more room like Tahoe said. Plus that way you can always trade one or sell one if you'd like to later. Or use one to cut frags from once it gets large enough and leave one alone and allow it to get nice and big. They should do fine either way though.
No need to cut the rock, just the toad, place anywhere you want him, gel super glue will help if there isn't a crack to put him in. They will stick them selfs in a day or two