Hi

sounds like a nice start to a tank.
Just want to maybe make you think of "semi aggressive - aggressive" fish - generally speaking fish termed like this are more intelligent and less likely to be a schooling fish they are also more likely to be a micro predator of sorts - as in would usually eat bugs and water insects. It is also quite relative, as in it depends what you keep as to judge how aggressive something is. So for example I have always kept American Cichlids which some would term as aggression but if you balance all the other tank members around this it rarely leads directly to conflict. For me using words like semi aggressive and aggressive to term fish simplifies it down far too much, in a well balanced tank that is well planned etc the levels of dominance, intelligence and interaction should mean that no one fish or group of fish actively displays consistently aggressive behavior as this is when you will loose fish at some point as one day it will just get too much.
So yeah... just wanted to put my thoughts on that out there.
Personally I wouldnt keep Bettas in a 30 gallon tank, due to how they are bred they are not suited to the currents you get in the bigger tanks they often get exhausted and its just not an ideal tank for them.
Anyway back to the tank in hand! 30 gallon tanks are a great size and can be quite flexible, like I mentioned earlier I love American Cichlids, they differ from their African cousins as they ideally want to be kept with other species in the tank to fulfill other roles.
For example with American Cichlids it is often best to have schooling fish with them this is for 2 situations, 1 - if you have a shy cichlid a calm school will act as dither fish, this is to show the shy cichlid that there are no threats around and it is safe to patrol mid water where as without the school it would be telling the cichlid that there are no small fish around which means there is something killing them there fore its best to stay hidden and out of trouble. 2 - The other use a school of fish has is if you have particularly tough fish together (though not in this size tank) and you need a school of robust shiny fish to distract the cichlids from each other and be able to take a slight hit when it happens.
In a 30 gallon tank I would mainly look at peaceful South American Cichlids. Things like Apistogrammas, Laetacaras, Keyholes, Checkerboards, Nannacaras, Cupids, Bolivian Rams, Blue Rams (if you went down a very specific route). There are some broad groupings in that list but you will find species within most of those groups and how they differ
Apistogramms are best kept in hareems with 1 male per tank and a few females, some species I think the females look a bit boring but my favorite is Apistogramma Panduro with their electric blue males and their bumble bee black and yellow females it makes for a nice contrast and they are quite easy to keep. Laetacaras are best kept in pairs but are probably the most peaceful in there and you could keep a few pairs in a 29g. Cupids would be a great addition to the tank, they are best kept in groups, 4 would do nicely in your tank some places state them as getting to 5 inches but that is the absolute maximum and most will stay between 3.5-4.5 with size differences appearing in the group as they grow depending on the hierachy.
Rams can be tricky, Blue Rams are hard to keep because depending on how they were bred dictates how they need to be kept and its very hard to know exactly where the ones your buying come from - ie are they close to wild or or are they a long chain of tank bred fish? If you like these the most make sure you get wild caught fish - they need very soft water and a ph below 6.8 and high temperatures around 28c. However by contrast Bolivian Rams are much harider and easier to keep - they do well in groups as well, they are quite gregarious fish.
Have a look at the names I mentioned and see which you like
The fish I would put with them, would be a school of tetras - maybe something like Cardinal Tetras, Pencil Fish, Marbled Hatchet Fish. You could also look at larger characins like Spotted Headstanders a trio would be nice. A school of Cories for the bottom something like Pandas or Triniliniatus are always nice. And maybe some whiptail catfish like LDA010 Red Lizards
Wills