Their bellies getting bigger after eating is normal, but if you notice them getting too big you might want to cut back on the amount of food. IMO
if it takes them 5 minutes to eat it all, it is more than they need. As for the oto, they like to eat things off the bottom such as algae wafers and shrimp pellets (and algae of course). Your neons will be just fine with the flow turned down some.