You'd need something stronger than topcoat for that many crystals I think. You could perhaps try using a very strong nail glue such as gel bond, but the weakness there would be glueing onto polish which will chip away from the free edge. I suppose you could glue straight onto the nail. I wouldn't expect them to last for weeks though, maybe one?
I'd do it on myself as I could do repairs easily but I'm not sure I'd charge clients for this as the length of time they'll last, people won't pay that much for & it's not worth your time!