Okay so I have an almost 4yo female ferret that I raised since she was a few months old. I was not able to have another ferret at the time so I gave her attention and love  and new toys all the time so she wouldn't get lonely. About a month ago I got a 3mo boy so she can have a friend. I've done short introductions in a neutral area and I'm aware that you're supposed to let them hash it out as long as it doesn't get too rough, but I'm worried that Esme(4yo) is too used to having just her, my boyfriend, and I in her business to let Pepper(3mo) in as well as she is not used to other ferrets. I have their cages side by side so they can get used to each others scent and be face to face with bars in between so Esme doesn't get too overwhelmed. I really just want my fuzzbutts to bond but I don't want to over stress Esme. She seems to have some interest in him but then sort of shuts down and occasionally hisses(which shes a little temperamental  and  hisses even when she's not very irritated) and of course Pepper is just overjoyed to have a friend as long as she accepts him. Any suggestions or tips are greatly welcomed as all I want is for them to be bonded and together finally. Thank you so much💖

Hello and Welcome to the forum!
I had a similar problem with my first ferret when we got our second. Bandit was used to being the only ferret and wasn't thrilled with Smokey. Bandit was over a year old and Smokey just a few months.  He moped and acted like he wanted to kill Smokey.  I asked a knowledgeable ferret lady what to do. She said to put cages side by side like you're doing but also switch their bedding so they get used to each others scent. After a few days I got them out for supervised play time. If it seemed too rough I separated them. Usually the rule of thumb is no poop no pee no blood it's ok. I still separated if if seemed to rough. Eventually Bandit accepted Smokey and they were cagemates the rest of their lives. Don't be discouraged if it doesn't happen rightet away. 

 
Fuzzbutts Rule!

Okay cool! After making this post I also realized that the baby, Pepper, is deaf as he is a blaze and has a neutral crest defect  so  I'm hoping that won't make things take too much longer since he won't be able to hear any hisses or anything from Esme. 

Hi and welcome!
In my experience older females can be real drama queens with new additions, ( female is always dominant) but it's the males that can be brutal.  Usually, the alpha female will establish dominance with newcomers pretty quickly, and the newbs are ok with it, with few exceptions.  Males will bully younger males for a few months.  Usually, the youngster takes over when he is big enough to handle the oldster. 
Or last addition was a 10 week old kit into a business of 4 year olds.  The alpha male was merciless to him for 2 months.  Then the tide turned when the youngster got as big as Yoshi.  Scamp picked on him for a few months.  Now it's al rough play.  No one gets hurt, but there is a lot of chasing and squealing, but no blood or poop.
 Good luck.

THE DOOKS OF HAZARD