Well in all honesty, you can really beat the game with whatever Pokemon you want, but there are so many options better than furret and ledian
Furret:furret is quite possiblely the worst rodent of them all. With its only decent stat being it decent base 90 speed and the rest of its stats being subpar at best. It makes a decent HM slave though. There are many good normal types available to you in soul silver, including miltank, Ursaring,ambipom and even raticate, that are better than furret.
Ledian: ledian is a similar case.its only remarkable stat is its surprisingly good base 110 sp def. also, its offensive stats (55 base sp atk is the higher of the two), are quite poor. But what it really comes down to is that ledyba is hopelessly outclassed by both Scyther and yanmega, who are both available to you.
However, you can still defeat the game using these Pokemon, so if that's what you want to do, go for it.