Will my adult male (large) rat be ok in a cage designed for guinea pigs?
The rat will probably escape, rats are fantastic pets but they are designed to fit into small spaces, if their head can fit through it it just takes a little squeezing and the rest of them’s out to. Try some chicken wire on the outside to make the gaps smaller. You also might want to consider getting two males as rats are highly social and love company, I have 4 unrelated males together at the moment and have had as many as 8 in the same cage (well a large converted hutch anyway), as long as they have space, males very rarely fight as they hate being alone. Think of the social structure of wild rats (they usually live in groups of hundreds). Hope this helps!