I'm sure that most of us have done what you have at least once,but the good thing is that you now realize that it's not the best arrangement for your fish,and indeed are trying to change it for the better.

Pristene water will be essential to keep them healthy,especially because the more they grow,the more waste they produce. You will also notice that it will take more water changes to keep up with it all.

They can live together for a short period of time as long as you're staying on those Params.The only thought I have to add is that the longer you keep them overcrowded,the greater the chance of them becoming ill or actually stunted. Good luck with the searching for the new tank.