Cichlids...there are a lot out there that will work in your tank size as the previous posters have menitoned, bare in mind that cichlids are not for the new fish keeper as their aggression can be a real issue...even on the cute little rams (gold rams are my fave, their colorings are just too pretty).
Somthing you may consider instead of cichlids are American Flagfish (Jordanella Floridae), they are killifish, yet this particular one is similar behavior to that of dwarf cichlids, or so I have read (getting one for myself next week).
With cichlids I have found that the best schooling fish to keep with them is tiger barbs (a school of 5 works fine). I have used the tiger barbs safely as a great dither fish for cichlids in the past and always recommend them.
When it comes to male bettas, there isnt much you can put with them that doesnt run the risk of him attacking, or it getting its beautiful fins nipped. However, I have safely kept a male betta with an African Dwarf Frog (ADF).
When it comes to the frog, they need easy access to the top of the tank, so the tank can not be that deep. They also will consume small fish (like neons) if aggrivated or hungry. The ADFs will eat the fish flakes that sink to the bottom, but that isnt normally enough for them...my ADFs main staple diet comes from ReptoMin Frog and freeze-dried bloodworms. I have never kept an ADF with cichlids, so I dont know how they would respond. ADFs are more active in groups of 3 or more, however one would be fine by itself. I have never kept ADFs in a tank larger than a 15 gallon, for fear of them being unable to reach the top for air.
Some cichlids like to eat the eyes off the corydoras, and I personally wouldnt keep them together. Of course most of my cichlid experiance is with the larger (6+ inch) cichlid breeds. Cory need to be in groups to be happy, I advice 4-6 cory. I personally dont like the idea of mixing cichlid with non cichlids, but thats just me.
Tetras. Depends on what type of tetra you want. Small tetras would be ate by cichlids most likely. With cichlids, you need larger and fast tetras. If you skip on the cichlids, a great schooling fish is rasboras (which are for a calm community...would probably do nicely with a female betta, ADF, cory, and other peaceful fish that would all fit nicely in your 25 gal.)
I would not keep ANY tetras with angels. To be honest, I think a 25 gallon is too small a tank for angels. When angels are full grown they can be as big as 10 inches top to bottom fins...which also are very attention catching for the nippy tetras. Even with a larger school of tetras nipping issues can and will still happen. Not that tetras are bad fish, you just have to watch which ones you get and what else you plan on putting in there with them.
Snails...not to familiar with as I havent had them, but I plan on getting some. I was thinking about getting a mystery snail, just one, as I heard they arent as quick to breed as others, and are great cleaners. The other reason I was looking into snails myself was as a food source if I choose to do dwarf puffers anytime soon...in which case I would just pick up a bunch of the ones at petsmart that over-populate quickly that they will give for free...as they are small enough for dwarf puffers.
I advice doing plenty of research on all they types of fish you are interested in keeping. Compatability and water parameters are what you will want to pay close attention to, as well as if it will fit into your tank size and how large of a bioload you are looking at...cichlids are very messy fish with a heavy bioload.
Most cichlids, tetras, and barbs, for the most part, can survive a cycle...but then again some breeds cant (I would NOT put an angel through a cycle). So start with a fishless cycle or choose a fish that can withstand the cycle. That should give you a good 3-5 weeks to figure out what to stock the tank with while it cycles.
Any questions or opinions, just ask. Good luck with your new tank, and welcome to TFK!!!