Exactly, good question Omorrokh Maybe first you could tell us Paul if you have any particular species in mind? But, remember - whatever the species - make sure they're all compatible with each other and not too large for the tank sizes you have. The size of a fish that should be taken into account is its ADULT size, not the size it is when you buy it. So it's best to RESEARCH each species before you buy it, both to see how large it gets and its general characteristics. And - of course - do NOT overstock.
You could have 2 angelfish in the 33 gallon tank. If you want to know whether a Betta can be put together with angels in one tank, Chickadee will probably have the answer for you. But in case you can't put angels with a Betta in one tank, why not have the Betta in the 10 gallon tank? Now, whatever other species of fish you add to the 33 gallon tank with the angels, and to the 10 gallon tank with the Betta, make sure that any species that you add are compatible with angels and Betta. Also, as I've said before, consider each species' adult size and do not overstock.
P.S. Are these US or UK gallons that we're talking about?
The Betta and Angels should get along fine. I've never had a Betta aggressive enough to outright attack anything other than another Betta, but to be safe, I'd have the Angelfish in there before the Betta.
You could get a few Corydoras for the bottom and... well, there are a ton of great small fish you could put in there.
I have to disagree, Bettas and Angels are a bad combo. Both can be aggressive, and they look similar. And actually, a lot of Bettas will attack or nip other kinds of fish. You have to be extremely careful when choosing tank mates for Bettas.
Sorry, Marc, but they are not compatible. Bettas and Angels will have at each other eventually. Bettas are best kept with little peaceable fish like cories and otos in 5 or 10 gallon tanks or maybe a 15 gallon if you are particularly generous. They may get along with a few little fish like some white clouds and I have heard some people have luck with platys but only in a 15 or 20 gallon where there are plenty of places to hide and remember that platies are very close in resemblence to female bettas so may not be accepted by some bettas. It all depends on the bettas. We have had owners post in the Betta Board who are having trouble getting their bettas to accept ANY tank mates.
Isabelll is the expert on what will go well with Angels, but I vote NO on Bettas. Other fish that do not do well or have been reported not to be good tank mates for Bettas are GOURAMIS of any kind - they are members of the same family and will fight until one or both are injured or dead, tetras - they are fin nippers and cause many a case of bacterialfin rot in Bettas which can be fatal if the Betta is not promptly treated in a quarantine tank with a good medication and scrupulously clean water; mollies, swordtails, guppies, platies, rasboras, or any fish with longish fins of any kind as they may be deemed as another Betta by the betta and attacked as such; Barbs as they are also territorial and aggressive and would be a poor choice to go with many other fish in a community tank. Good tank mates for Bettas are few and far between and include Corydorus catfish of many varities and Otocinclus catfish of a few varieties and Mystery Snails (one of the few kinds that are either male or female and do not reproduce unless you have more than one in a tank) [these are NOT Apple Snails]. I have also heard that the Dwarf African Frogs can be a good choice but I have not had personal experience with any of the above except the Otocinclus catfish and the Mystery Snails and I can say that they are excellent choices. I wil also say that there is a real problem to have the Betta in the tank first and then add the other inhabitants. They see this as an invasion of territory that belongs to them and it is not a pretty sight and they can kill those who are added to the tank. It is also a fallacy that you can have male and female Bettas in the same tanks - this is a definite NO-NO. The male will attack the female unless there is a spawning in progress and when the spawning is over the female must be removed IMMEDIATELY.
This is just a small example of the many suggestions of the care and loving of the Betta. They are a very loving and rewarding fish to own and but I would never, ever recommend them in a community tank.