Mixing species is a definite no-no. Keeping several of the same snake is even not a great idea.
=typically snakes are solitary animals and do not enjoy eachothers company
=they have their own host specific bacteria, which while it is fine for them, can be harmful to other species (in some cases even fatal)
=these animals may have different husbandry reuirements
=these snakes may have different habits
=possible risk of hybridization
Then there are the reasons not to house even same species...
=They are generally solitary animals and the stress from being forced to share its space may result in behavioural/feeding problems
=They could fight or have accidents
=If one is sick the other will contract it if it is contagious
=Its impossible to monitor them individually... if one defecates/regurgitates you won't be able to know who it was?
=Have to do seperate feedings
=Females can be seriously injured (potentially fatally) if bred too early