As it has been stated many times in the past,it greatly depends on the "birdonality" of the fid's. Having had parrots for over thirty years,I have two. Smokey (TAG) was the first then about a year later Amy came a long. Smokes was about 1.5 years old at that time and Amy was just four months old.
Amy's birdonality from the start was great,she wanted to be buddies with Smokey but Smokey wanted NO part. Thinking back,Smokes was probably jealous of Amy since she was an only bird from the start and she made it known she DID NOT like Amy

If they were out together Smokey did everything she could think of to torture poor Amy

She'd sneak up on Amy then pull her tail and run back to her house LAUGHING

One time years ago Smokey went up to Amy and they went beaky to beaky "kissing". I thought to myself that Smokey was finally accepting Amy, but I was sooooo wrong. Smokey BIT Amy's tongue so hard that Amy screamed in pain and I had to rush her to her CAV and they put five stitches in her tongue. That was the last time I let Smokes get close to Amy.
A few years ago Smokey flew off to Rainbow Bridge (she was 28 years old)
I then adopted Jonesy, A goffin 'too. Jones was smaller than Amy,about the same size a Smokes was,and Jonesy and Amy got along VERY well,where they would hang out together,preen each other etc. Unfortunately I ended up rehoming Jonesy ( long story) about a year later.
Now there is BB the Cockatiel. The Beebs is MUCH smaller than Amy,and Amy wants to be best buds,but I think BB is a little intimidated by the Green Dinosaur's size. Amy is constantly climbing Beebs house to hang with him! She has even gone into his cage,to play with HIS toys or even nosh on his food!
BB is flighted and when he gets aggro'd with Amy's antics he'll just fly away,usually coming to me. Amy doesn't fly.
Sooooo...long story short, I believe big and small fid's can have a good and happy relationship. It just depends on the individual

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Jim