I have 3 chinchillas. What the others have said is all right.
They are wonderful pets, but certainly not for everyone.
They are 'shy' animals and often don't like to be held.
They can die from being in temperatures too warm. I have a marble slab that I put in their cage for them to lay on which helps from overheating if the power goes out.
I definitely would not put them with guinea pigs.
They do need dust bathes. I give mine a dust bath a couple times a week.
They are nocturnal and very noisy at night.
They are quite messy. The dust gets everywhere and their fur flies everywhere too. Vacuuming often (daily) is a must. I have a shop vac that works well.