For one, or maybe even a pair, it's completely fine but it's more likely to reach 6 inches than 4. What you're forgetting is that your acei has actually lived more or less on its own other than the cories and pleco. That's why I warn about it being overly aggressive with new inhabitants because you've already seen what happened when you put a paradise fish in with it. The mbunas in your friend's tank will be used to having to share their space. I just think it could be risky.
If you want to try a mbuna tank in your future 30g, by all means do it. I just advise that you stock it lightly and keep a very watchful and still eye on the tank when you add new fish to look for any aggressive behaviour. Also, add your acei either at the same time as, or after your new fish. This will make it more difficult for him to immediately take control of his new surroundings and set up territory. If your acei continues to be a problem I'd replace him.
Really, don't feel like you can't ask for advice - we just want what's best for your fish, just like you, whilst giving you the best experience and advice we can. So don't feel like you have to keep quiet. Ultimately it's your choice what you do but members of the forum, particularly those more experienced, are very passionate about fishkeeping, and we will voice our opinion if we don't agree with something, just as you should do the same.
So don't be driven away, questions keep the forum running, we all started somewhere and sometimes it takes a bit of hard advice to get us on track as a responsible fishkeeper.