So, I have a 1.7.10 server in offline mode and I want to know if client side and server side use that mod everybody will see theirs ''new skin''

Players will be able too choose their skins?

If they have their new skins in their minecraft account, yes.
Otherwise, you will need a custom skin server that supports uploading skins by users.

ok but cracked clients, will be able so theirs new skin?

Yes, the mod does not check accounts.
You will need a custom skin server that supports uploading skins from your users.
I'm not gonna help you in setting up such custom skin server.

If you are enjoying Minecraft, please consider buying it.

I already bought minecraft a lot time ago !

No problem I found a better mod

okay