Before you try too hard potty training, let your rat (hopefully soon to be rats!) figure out where they want to go to the bathroom first. Then you pick that spot to put the litter pan in.
See if explaining to your mother that the petstore lady didn't know what she was talking about helps. Show her websites, like this one, that say that rats need friends. A lot of people, even those that work at the petstore, think that rats are basically hamsters with funny looking tails, and some hamsters should be kept solo. But that's not correct for rats at all- ask her if she thinks cats and dogs should be treated exactly the same, and explain that rats need the right kind of care, too. But be nice about it, of course.
It's never too late to give your rat a friend, just make sure you go through the proper quarentine and introduction steps. But if at all possible you shouldn't wait for four months, that's a long time for your rat to be alone! Every ten days is a year to rats, so that's like making your rat wait more than 11 years for a friend! Yikes!