Listen, I understand your need for a new pet, Hell I want a new pet too (a snake ;3), but if I was you I would wait a little. Your parents only want the best for you and I m sure you re really grateful for them allowing you to keep this many pets, but you have to understand your parents point of view, you already have a lot of pets and are you sure having another would really be beneficial to your other pets? Are you sure they would all get along? Do you really have enough time in the day too look after them with all your studies, work, school and boyfriend?
I m truly sorry to hear that you suffered with depression and it was super nice for your parents to gift you a horse to try to deal with that. Now you asking for another pet might seem like your abusing their kindness, maybe they might not think that other people definitely might, especially people on the internet xP.
You re clearly an animal lover but I m not sure taking on another pet would truly be rescuing them and giving them the life they deserve, even if you think you can do that. A lot of people like to rescue pets and I can see you re one of them but as I ve said before, I would wait until you re much older and you can properly care for yourself, settle down and get a job.
Look, I m taking you at your word here and many people would not, a little advice, don t correct yourself and add loads of information when trying to defend your point, it seems a little fishy to others xD.
However, if you ve thought a little more about it and still want to get a Cat/2 guinea pigs, I would try to show your parents that you really are up for the job and that they won t be luggaged with having to care for your pets. Clean around the house and do lots of chores so you can show them how hard-working you are and that you can take on more responsibility. Try and get really good scores on all your tests and if you have a subject you ve been doing bad in try and get an A* in it so you can show your parents that they don t have to worry about your education! Lastly, I would take them out to a nice dinner and get them some really nice gifts for their Birthday/Anniversary/Christmas to really butter them up and show how thankful you are to them!
One last thing before I go, I know you love your pets a lot, but don t focus your whole life around them, go out and have fun, hang with your friends (not your pet s even though we all like to think of them as friends!), show some more attention to your boyfriend. If worse comes to worse and your parent s don t let you get another pet, maybe ask your boyfriend to get one and you can go to his house to be with it (Although, that might be a bit much to ask of him!) :3
I wish you luck!