Encryption on wifi adapters

I've been having trouble with my old router which used WEP, so have invested in a Netgear N150 from ebay. It appears to be working fine, so I thought I would set it to the Mixed Mode WPA + WPA2 for wifi..

I have a network (workgroup) of a few cabled machines running various legacy MS OS's, but also have 6 laptops running XP, Vista and Win7 to enable software testing. These all connect wirelessly.

There is a complication in that I am now also fighting CentOS, but the following is with that machine switched off and so its effect is no longer a problem.

I am confused by the laptops' settings. If I have them set to WPA2, I am offered the choice of TKIP or AES encryption. It appears that if I set some to TKIP and some to AES, they don't see each other over the network, so I have set them all to AES. It has taken a lot of time to discover this.

The question is, what happens when I connect an older laptop that is pre-WPA2. Should I set them all back to match TKIP or will the Netgear cope with one encryption on WPA and another on WPA2?

I wish I knew why I always find things so difficult!