hi,
Reason I mentioned 8 minutes is to keep noise and amp glow low.
try taking darks at iso 800 with your canon from 1min to 15mins. have a look at where you start seeing the amp glow which is a reddish glow in a corner and at which exposure you start seeing random colour noise, suggest trying this when the ambient temperature is below 20.
I'd suggest another set with iso 400 as well. with a lower iso, you can go longer at the loss of details.
the meade dsi is a ccd with a sony chip and would have lower noise than the dslr and from what i could find, the QE or quantum efficiency is above 60% for the mono. which DSI do you have?
the 8 minutes was only a suggestion, you'll need to experiment and see which length suits your imaging with acceptable noise.
general rule of thumb is that if the outside temp is low, that is below 20, then long exposures with an uncooled dslr is fine, so i'd suggest varying the length based on ambient temperature.
you could experiment with shorter exposures and higher iso or the reverse.
unless you try a number of combinations, its hard to speculate which is ideal as there are several variables.
The QE of dslr's is not measured in the same way as mono CCD's from what I've read but the comparable value is in the 30% range compared to CCD's which are generally above 60%.
have a look at the dark frames here. the 600d and 450d are relatively low noise. these darks are at iso1600 though.
http://dslrmodifications.com/T3iReview/T3iReview.html
don't worry too much about length of exposures, depending on your skies, 5 to 10min subs but a lot of them will yield good results and for galaxies, you will need reasonably dark skies.
the other option is to look at one shot colour ccd's but i'd suggest taking it one step at a time, experiment with the dslr and then move on.
Cheers
Alistair