no none of the moshlings grow bigger
No, you can not derss up Baby Rox on Moshi Monsters. You can not dress any of the moshlings.
You do not need a secret code to get a Moshi Monster. You get your Moshi Monster when you sign up for your Moshi Monsters member account.
There are many members who have all the moshlings. It would be difficult to keep an up to date list because Moshi Monsters continues to add new moshlings. Just check out member's Moshling Zoos when you visit their room to find out how many moshlings they have.
After you have planted the seeds in your garden you just have to wait for them to grow. There's no way to speed up the growth of the seeds into plants.
If you mean moshlings you plant seeds in your Moshling Garden. If you have a free membership, there are only certain moshlings you can get on Moshi Monsters. If you mean monsters, you can only have one monster on your account, the monster you chose when you signed up for your Moshi Monster account.
No, you can not feed your moshling on Moshi Monsters. Feeding Moshlings isn't part of the game and could end up costing a lot of Rox if members had to feed all of their Moshlings!
You can not speed them up, it takes 6-7 hours of growing. If you stay in your garden, they do not grow! Make sure that your Moshi Monster is happy and healthy and the seeds will grow faster.
Playing the Daily Challenge, Washing your Moshlings.
Yes, you have to have at least a free Basic Membership on Moshi Monsters to collect moshlings. With a free Basic Membership, you will only be able to keep two moshlings in your room. With a paid Moshi Membership, you may have up to six moshlings in your room and can access your Moshling Zoo to see all the moshlings you have collected.
If you have 999 Health and Happiness on Moshi Monsters, the seeds in your Moshling Garden will grow faster. Keeping your monster feeling well also helps you earn points towards leveling up.
Free Basic Members can have two moshlings. Paid Moshi Members can have as many moshlings as they want. They may have up to six moshlings in their room/house plus they can have all the moshlings in their Moshling Zoo with one or more of each.
There are moshlings which have not been released yet on Moshi Monsters, and those would be the only ones that no one has. Some moshlings show up in the Moshling Zoo before they are released, so no one would have those moshlings until they are released.