Using the same wireless access point for home lan + filesharing as well as for a free public hotspot

Is it really possible to set up something like this with decent
security? My idea is-


[Modem]--[RedHat firewall/router]--[wifi router]--[2 wireless users ]
I I [with access to ]
I I [ shared files ]
[1 wired user ] [ any public ] [ and a maximum ]
[with access to ] [ users- no ] [ 100% of total ]
[ shared files ] [access to ] [internet bandwidth]
[ and a maximum ] [anything ]
[100% of total ] [except the ]
[internet bandwidth] [internet and ]
[only 15% of ]
[total ]
[bandwidth ]

So even if no-one else is connected public users only get 15% of my
total bandwidth (shared equally between themselves) whereas if one of
my secured users is online they get a full 100% of my available
bandwidth.