Yes, they need a big cage, get the biggest you have space for. Chinchilla cages are great provided they don't have a plastic bottom (some degus chew straight through those!).
It's now unnecessary (and not recommended) to feed degus chinchilla/guinea pig food. There are now a few degu-specific foods on the market that you should get instead. I would highly recommend SunSeed's Sunscription Degu Diet. It's timothy hay based and has no sugar. Rodent blocks aren't recommended at all so no, you don't need those. Degus are herbivores so their diet is closer to, say, a guinea pig than a rat
Yes, you want to avoid molasses, as it's a form of sugar. You also want to avoid cane molasses, cane juice, cane sugar, corn syrup, etc. Those are all just forms of sugar. Also avoid foods with corn, degus have a tough time digesting corn and it's a very sugary food...something to avoid with animals prone to diabetes.
Yeah, I'd give them some time to settle in, unless they're already really used to handling
. Start by letting them come out on their own, don't pick them up or restrain them in any way until they're used to you.