I am not being personal here so do not take it as such but it bugs me when people say they want to get this or that fish "way down the road" cos 99% of the time they will get the fish a week later regardless of all advice given against it.
Your poor little Gudgeons have had so much stress and hassle since they arrived that they need time and space on their own to settle down properly. They do not need more intruders of any kind to bounce into their home upsetting their already fragile state of health or irritating their sense of territory.
I am sorry but your decision to buy Gudgeons was ill advised, they are very particular fish who need very careful habitat care. They are not beginner fish, tbh I am not even sure they are intermediate fish due to their very precise needs.
Had you researched them properly you probably would never have bought them as they are extremely tough fish to house in a community setting. You may well find that you end up with just the Gudgeons and nothing else in the aquarium. When choosing fish it is absolutely vital to check their habitual needs, not just their water chemistry needs, not every fish is good at being with other species, something I think you have found out the hard way with these fish after they showed their darker and more aggressive side more than once already.
They are lovely little fish, enjoy them and watch them mature, learn about them in all aspects not just the water conditions that they need....they deserve a peacefull and stress free life, not one where new fish are added now or later on "down the road". I shall be blunt....its not their fault that you didn't do all your homework, so they should not suffer for that.
Slow down, research everything....and if mum is there pushing to get more fish cos looking at just the Gudgeons is boring or whatever, tell her no cos the health & welfare of those Gudgeons is the most important thing right now......the LFS should not have sold them to you without you doing substantially more homework on their needs, at the end of the day those fish were just another part of the LFS profit and nothing more than that.
... Cause before I got the fish I "cycled" it by throwing some fish food in there and leaving it for three days 
... I also made sure every week to change out the filter cartridge... Yah, I was a fish keeper's nightmare... I used two meds at once and thought my snail was alive when I used the ich cure... The finrot med was kinda working tho... Yeah it died in less than a month... Now that I'm thinking about it it had to have died in less than two weeks...
