Remember that the iPad is about two bricks short of a full load, means that it is limited, the reason for that is storage and RAM, only so much to go around.
Need room for the OS, apps, memory, data for networking, email and a host of other things.
You start with a full featured app like iPhoto
and you have no room to do anything with it.
The order the photos appear in Photos is the order the slideshow will be in, you can select transition, to play or not to play music and what music.
To do what you want to do it has to be done on a computer and then ported to the iPad.
That is just the way it is, I didn't write the code and cannot change it, I use it lie everyone else.
Would that be a useful feature, yes it would, I would like it, but.
Hope that helps.