definitely too much! a 5 gallon can have a divider and hold 2 bettas. but, it's maxed with two (2" adult size fish). Now, talking bettas - the males especially are territorial and won't take kindly to other moving critters in their 'home' space. Those cories (which in a larger tank will be well loved) ... 5 gallons - their zippy behavior is going to freak your poor betta out. If the betta doesn't kill the cories, then the cories will stress out the poor betta. 5 gallons is not as big as you think it is.
I'll give you an example... imagine you're about 50 years old, and used to having your own space. you don't much like people coming to visit, actually - you prefer your own space to the point that you don't ask anyone to visit and hope that the neighbors don't notice your little house hidden behind all the trees. (Now, that 50 year old - is your betta) - now, imagine that suddenly the neighbors grandkids move into the neighborhood and decide they not only like to play in the grandparents' yard, but also in your yard... and, not only that, but they make themselves comfortable in the 50 year old loner's kitchen, bedroom and bath... without knocking... or asking. Now those visiting grandkids of the neighbors - well those are your cories.
You think the betta will get used to them? Or wish them gone already?
Now - don't get me wrong - communities 'can sometimes' work. yes siree. But not too often with betta males in 5 gallon tanks. (In a ten gallon - possibly, but a lot depends on the personality of your betta. In a 20 gallon - you'd even improve your chances... )
I hope this will give up your hope of risking these fish, thinking that your betta boy really wants company. Trust me - he doesn't.
TheJewels... 20 g H plantedsorority - 4 princesses & 3 zebra otos
Ladies of Camelot - extended visit of Rome - 7 pretty girls & 7 cories - 20G Long.
Gawain - hM (2.5 g planted tank)
Lancelot - hM (5 gallon tank)
Roland - CT (5 g)
Galahad - deltatail (15 gallon divided)
King Richard the Lionheart- king betta (12 g Camelot)
Tristan - VT- (15 g divided)
Robin of Locksley (20g H w/threadfins/neons)