new 802.11n router + connecting to adsl2 router

I've currently got a Netcomm NB5Plus4 ADSL2+ Modem router w/ 4 100mb ethernet ports built in,
and I'm going to buy a TP-Link TL-WR941ND Wireless N Router (with 4 100mb ports, and WAN port) to add wireless capability and extra ethernet ports to my network.

There are several things i'm not sure about -

-So if i have DHCP enabled on by Netcomm router, I just connect the WAN port from the TP-Link into one of the Netcomm's ethernet ports, and it should work? (ie, internet access, computers can see each other)

-If i have mixed devices - some 802.11n, and other 802.11g, can they all co-exsists at their respective speeds, or does the entire network throttle to the speed of the slowest device?

-I'm pretty sure that for the network security, that DOES get downgraded to the capability of the lowest device? (e.g., if all my gear uses WPA2, but i connect a computer using WEP, all the devices fall back to WEP)

I'm also replacing the router's 3x antennas with 5dbi omnidirectional ones, so any issues I should know about?

Thanks in advance.

Well In my house I have 2 computers running off 100mb ethernet, a laptop with 802.11n, and another laptop with 802.11b/g (should have paid $30 more and upgraded to N at the time ) .

Shouldn't have any WEP only devices anyway, unless people come over and hook up their pdas/phones - some of the new model samsungs/motorolas are still WEP only...

Anyone else know the answers to the other questions?

Thanks