digital photo's using eye piece projection question

I have a 5 year old Meade etx 90 go to scope and have had a great deal
of enjoyment using it. I'd like to take some photos of a few things
using a canon digital camera using eye piece projection.

I have a few questions,

1. what is the highest magnification eye piece that can be used with
this configuration?

2. should the eye pieces be the highest field of view possible?

3. Is there any software available that will allow me to sum multiple
photos into one photo that simulate long exposures?


Thanks any hints or tips welcome.