In a 10G with good filter and regular water changes, you will be ok with 8 guppies the molly and maybe 4 corys as the corys ar bottom dwellers and the others mid and top.
This is providing you get rid of the plec (Big waste producers)
The filtration is the key...with regard to space i would say around 5 personally just to make sure they don't get stressed at the amount, but the filter needs to be very good for that amount of livebearers as they are quite 'messy' fish!!
I'd say it depends on whether your haveing a same sex group or mixed
If same sex (asumine that it will be males) you can easily have 6/7
If mixed then i would go a trio (1male 2female)