It probably wouldn't be a good idea,since guinea pig cages have huge bars that rats would easily escape from
I've seen modified C&C cages used for rats,but they have to be meshed over which can be annoying but would be a great idea if you were willing to put in the work.You'd need 20 grids if you made your own base without using grids and adding solid flooring,and if you bought the grids from Sears that would cost around $23.The mesh would be about $16,then there's the cost of zip ties which is minimal at around $3 or less.If you already have wire cutters or zip ties,that would make it less expensive,plus those items and the materials are worth the investment.