The 5i/8i mount (the motorized base and arm) is almost identical to the 6SE/8SE mount. The 4SE/5SE mount is similar - the mounting holes are the same but the round part of the base is a different shape. In any case, a wedge that works with any one of these will work with the others. This is the manual for the current SE/Evolution manual:
As you can see, it attaches to the tripod using the same bolts used when you put the mount directly on the tripod. One difference is that the older 5i/8i tripod did not have the indents for the "registration pads" but that is a convenience and will not prevent the use of the wedge on that tripod.
That said, a wedge will allow slightly longer exposures but not dramatically so.
As you have discovered, the corrector plate cover is different. Pretty much any cover for a pre-SE 5" SCT should fit. Otherwise, I'm sure you will figure out a homemade cover.
Any other accessories you have questions about?
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"