well, If I am not mistaken, if your car wasn't originally r134 then it will ruin your system. I think they started using r134 in '95 or '96, somewhere around there. I cannot answer on the psi question, for I have never done that before.
The difference of SSE and SE is that SE is the lower end trim model of the two [I am pretty sure the base trim model is LE]. SSE will have buckets in the front and a bench in th eback [leather of course]. There is a better Driver Information Center [the SE has just a few lights and and a few less features warning you of door ajar and low washerfluid and maybe one or two more functions] SSE has a digital display, displaying all the SE has plus a service reminder, to remind you basically when the car needs service [user inputted, it just counts down milage] There will also be a better audio system [probably just a better head unit]. Thats really about it, if anyone would like to add to that please do.
'89 SSE - GX3 -190k, 155k on body - Back for another ride.