I would offer both loose hay and cubed hay. Personally I think loose hay is better for a chinchilla's teeth and diet as it is less processed and requires more back teeth grinding. However, both are perfectly safe to offer. My girls I got at 8 weeks and they ignored the hay cubes until they were about 5 months old but munched the loose hay from the very beginning. Just make sure that the high is a good quality, no mold, etc. Timothy hay is most commonly used but the botanical mixes and even alfalfa hay is safe. I offer my girls several different types of hay and allow them to eat it freely. The largest portion they get is Timothy since it is the cheapest and easiest good quality hay to get ahold of here. Make sure you also have your new baby on a high quality pellet. If you are getting it from a breeder or rescue they should give you at least a small amount of the pellets that your new chinchilla was on with them. Then you can slowly get them use to whatever pellet you choose to use. I have handfed my girls both pellets and hay to help with bonding. They are just now old enough for treats but have yet to be offered any. They get a supplement, hay, water, and Oxbow pellets and an decent amount of wooden items to chew on.