Sorry to hear about your guy.. you say stuck in it, what did he get stuck in?
If it was the filter then you could put panty hose over the intake portion of the filter to help prevent the fish from being sucked towards it. Normally filters aren't strong enough to hold a betta there, unless the betta was already weak or passed on.
8 gallons is too small for a sorority- the bio load would be too great, along with not enough room for them to feel comfortable.. a 10 gallon is minimum for a sorority (and multiple fish) and even then it's pushing it as females prefer and naturally don't live together. So by having a smaller tank, it's giving them less room to get away from one another and some may feel "trapped".
Sororities are ticking time bombs.. most of the time they will explode, so to help give you the best shot at having it not explode too quickly, or too hard then a large tank fully planted densely is required.