Agree, at that age she should be eating solids. She probably nicked the other cats food as was starving hungry.
She WILL ahve a bad tummy as the adult food will be too heavy for her to cope with, especially if she hasnt eaten before.
If she still has diorrhea today then I would suggest you get her to the vets. Diorrhea and sickness in pets can be bad very quickly.
You can't really make her drink more than she wants, at 6 weeks old she's going to sleep a fair bit anyway.
She really ought to be on kitten food by now, try giving her some dry kitten food until her tummy clears up, if she can manage to chew it