you can do anything in Unity. What the problem is, is how much effort you put into it. Now, Character creation: you can do this 2 ways.
The first involves setting player prefs to store their information such as clothes, name, character model, hair-do… etc, and telling the server to look for it and applying it so that other players see you how u created it. This can be used for anything really not just their appearance, for example: stats, money, deaths, etc. 
The second involves creating a database and when a player registers the data is stored in the database. Next time they log in, the server requests the information from the database and sends it to the clients.
With both these solutions there will be security flaws.
In the first solution (playerprefs one) the player can edit these, granting them access to change any of the variables you originally saved, ie: appearance, money, etc
With the database, it can still be hacked, the best way to keep it safe is to use the server to “hand-shake” the client to the database. This means the Client asks the server to ask the database for the information, which then gets sent back accordingly.
Now, that was the technical part, the next part which is really the first part is the models. You should have your basic model mesh (say a male and a female), and on their prefabs have empty game objects in various locations around their bodies. You can then apply objects or clothing (hats, shirts, shoes, whatever you want) to the location of the empties. Doing the empty game object solution will mean you can still use animations and the empties will follow them.
As for the slider idea, you could do that but youd need a lot of bones in the face that move when you use the GUI. This is possible but could give some weird results. As for the scaling idea, that would be easy!
Side note FYI (just in case u didnt know):
Latency is a big issue for multiplayer games. So you will want to create objects that do not have too many polys, you can fake detail with normal, bump, spec maps.
Unlike single player games, its not just about conserving computer memory — its also about BANDWIDTH and memory leaks being plugged!!
GOOD LUCK!
If you need anymore help let me know. Id be glad to. I understand the struggles of multiplayer games, and have only really gotten on my way with producing something worthy with multiplayer functionality.