redirection and nat

Hi,
In my friend's office where I am trying to put a Linux server, I am facing some problems.

They have connectivity from 2 ISPs thru' Router A and Router B. Now in office people from 192.168.10.1 - 192.168.10.7 use Router B to reach internet and the rest Router A.

I want Linux to be put in such a way so that it accepts connections from all machines and redirects them to router A and B. Before this they will be checked against some rules like whether they are allowed to access external SMTP or not etc. Is this setup possible?

How will I use redirection here? I can redirect for a particular port but am not getting the syntax for directing any package to particular router's ip.

Do I need NAT here? If so, kindly tell the logic I have to use this with NAT. In NAT I have to specify an interface like,
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

But here I will be using 2 interfaces. What will I use then in above rule? And what should be my default gateway? Does it bind to a particular ip? BTW, I will be using squid too here. Please remember this too.

Thanks a lot and eagerly waiting for any input on this.

With warm regards,
-Payal