The opaline won't work with the betta - especialy if the opaline is male. The angels are also likely to kill the guppies.
As far as an algae eater goes, the siamese algae eater would be a good choice but you could also get some otos once the tank is mature. Besides an SAE, oblongus, false SAE and flying foxes will also eat algae but vary in terms of size and aggression (the FF is the most aggressive but not realy by much - it's also generaly the largest and most colorful and probably the worst out of those as an algae eater). Any of these should get along with what you are proposing except, perhaps, the guppies (but they're not a good idea anyway).
The american-flag fish is another fish that could work and will eat algae quite readily. It MAY get nippy with the angels or guppies but should be fine - a trio would be the best choice so they'll occupy each other.
Apple snails will also eat algae. You don't realy need an algae-eater anyway; just keep from over-feeding, make sure you aren't over-doing the light or over-stocking, use an algae scraper when necessary and keep up with water changes.
