can it be flashed to have straighttalk service
Search the forums please, There's numerous threads about straight talk and the answers are there.
On another note, Ya. Optimus S is not supported by ROM Manager at all. Usage of it varies, but most brick or semi-brick. It's always a good idea to research before you install something that requires root (such as ROM Manager) to ensure it actually supports your device. There is a nice replacement for ROM Manager coming along called GooManager and we actually have a recovery that supports it (Drew's Cannibal Recovery, which adds a few features upon CWM Recovery).
The Play Store has a way to define compatibility, but sometimes the criteria of the compatibility is not what the developer wants (for instance, I'm not sure if you can blacklist a certain device without blacklisting a whole set of features, example, ARMv6, Armv7, or Tegra only. If it's possible then it's perhaps more of a laziness on the developer's part for not publishing it with the correct incompatibilities.)
A lot of these threads (mostly the ones at the top) however will help you with starting fresh and rooting from there. If you want a custom ROM, we have multiple GB ROMs, One ICS (Technically two, but ParanoidAndroid is rather unstable at the moment, I'm gonna try re-porting it soon, I would attempt today but I don't have my netbook on me to begin) and one JB.