1) Yes, especially a baby. 20gallon longs are a bit better though 
2) For babies you want o use something they cant eat such as papertowels, newspaper, shelf liner, linelium or indoor outdoor carpet. I used paper towels for a long time but really hated it because the crickets crawled under them and they looked boring. I recently switching to indoor outdoor carpeting and have been pleased so far. I clean the poopies every day and scrub the carpet once a week. I know there are also a few die hard shelf liner fans here!
3) Variety is ideal, although plenty of people have raised leo's on crickets and meal worms without a problem. For variety you can add Silkworm (the best feeder insect nutritionally) waxworms, butterworms and roaches. Supplement with a vitimin powder once a week and with a straight calcium powder several times a week when they are babies. Less supplementing is needed as they get older.
4)there are ton of breeders who post on this forum, look in peoples signature for their links. Generally i advise against getting leo's from petco, but tons of people do it and i have done it myself. I will ssay if u buy one from them get the fattest healthiest looking one. Try to get them to feed the lizards so you can see which ones eat. And take a fecal to the vet immediately when you get them home. Petco has a 14day guarentee on all their animals so if you go to the vet to get a fecal and there are parasites petco should pay for the deworming. Make sure you talk to the manager first though because for the guarentee to be activated you have to take them to the vet that petco usually deals with.
Make sure to quarentine any new additions away from your crested.
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0.1 Bearded dragon
0.1 mexican kingsnake
1.0.2 Leopard Gecko's
0.0.1 Rose Hair Tarantula
1.0 BTS
0.0.1 Reverse Okeetee Corn
0.1 Bullmastiff
4.1 Cats