Will an unlocked PEBL work on cingular?

I bought one on ebay. Its unbranded. and brand new.

Yes, atleast the actual phone part. You may need to setup the MMS and MediaNet setting on it which is usually done here: Cingular | Web Tutorials
...but the Pebl isn't an option and not sure if one of the other phones settings would work. You may need to call Cingular CS and ask them about it.

Unfortunately, that link doesn't have settings for the PEBL and if they can't help you over the phone you'll have to use the trial-and-error method of testing the settings for similar models. I can't make any promises, but I'd bet the settings for the SLVR or ROKR will get that PEBL setup for ya!

Cingular Tutorials - Settings Configurator

what categorie would i be looking under?

For MMS - Picture Messaging
For web access - Media Net