Do not thin them out now. Autumn would be the earliest I would do anything.
Actually, after a "chop", more correctly, drastic pruning, You can wait 1 to 2 years before doing branch selection. During the waiting time, draw out, your plan for what you would like for these trees. Choose the size you want, style. You will need a plan, and as they are your trees, you need to make the choices.
Also, you should acquire potting media, the "rocks" you mention above are probably pumice. Or they look like pumice. Unless they are crushed granite. Also you will want to begin shopping for future training pots and eventually display bonsai pots.
Nice starts, You have nothing you need to do right now, but many things to research, drawing out your plan is the most important. If you can't draw, find photos of what you'd like the future trees to look like.