Yes, Lot's of water changes. My husband always says didn't you just do that?! And I'm like, yes dear

And I hear you on the husband and spending money on fish or fish stuff. I have the same thing here
Lot's of live plants in a fish tank are a good thing. Provides places to hide, places to eat, and helps keep water quality good. Bettas don't usually fight with other fish (at least in my experience) however I do keep one in my feeder guppy tank kinda as a form of population control. They will eat smaller fish and shrimp (learned that they hard way) lol
I have had bettas with lot's of other types of fish and usually if there is a problem it's the betta that is getting hurt not the other fish. If your bettas have long flowing fins you need to be careful with the intake of your filter (if your using hang on the back type filters) I usually cover the intake with a sponge.
Cichlids are cool looking fish and pretty hardy too. You just have to be careful what type you get and how you house them. My very first cichlids were the pseudotropheus aka bumble bee cichlid. They get big and as they mature they are aggressive (to each other) I had to rehome some because they were fighting (as in lip locking) My next cichlid tank had frontosa, daffodil brichardi and electric yellow labs. The fronts were my favorite but they get big, especially the males. My brichardi had tons of babies and were only aggressive during mating time. The electric yellow labs were fairly docile and beautiful color.
The only reason I got rid of the cichlids is that I had too many tanks and need to downsize. I couldn't get rid of my goldfish or the bettas so the cichlids went. Some day I would like to have them again.
Well enough of my rambling.....Happy betta shopping! Can't wait to see what you get
Oh and as for feeding while your away. How long are you going to be gone? If it's only going to be a week I would just leave well enough alone and don't feed them. If it's longer than that I would try and premeasure food and mark it with a date that you would like it to be fed. Maybe you can use something like a pill organizer. It will give you several small compartments to hold each feeding.
I have not used the white feeder blocks but I do use the gel feeders in a foil tin. I have not used it as a vacation feeder but as a way to make sure that my apple snails get their fair share of food.