If you are going to be using subinterfaces on the PIX, then you need to make sure the port it is plugged into on the switch is set for trunking. Otherwise, the PIX won't be able to see all the VLAN info coming across. As for setting up the subinterfaces, you should be able to find out how to do it with a bit of Googling. If that doesn't work you could always call the TAC.
It looks like you have the one of the first verions of IOS on the PIX that supported trunking. With 6.3 IOS though I don't see a way to configure subinterfaces on the PIX. That link should be able to help you do what you want.
Heh, yeah the PIX command set is completely different from a routers or switches.
I think the biggest problem you'll have is getting the PIX to work the way you want it to. A PIX isn't a router and shouldn't be used as one because of how it handles packets.
If you can try to find a cisco router that you can configure the subinterfaces on and setup the vlans accordingly. Unless you can download the new IOS image from Cisco for your PIX. I don't think subinterfaces are supported on pre-7.0 IOS.
Also the "switchport mode trunk" command is used when you are stacking switches and have vlan's configured on multiple switches.