Welcome to minis! I think it will depend on his attitude and size. If he is still a bit stallion oriented even though he is gelded, I would be careful about putting a small or young filly in with thim. If he is very calm and truly calm after his was gelded - I would still be careful about the age and size of the filly and introduce them gradually. Geldings can do well with mares, young colts and some fillies. They tend to play and if they are too big could accidentally hurt a horse that is much smaller. We have a small older mare in with our gelding and she bosses him around - but they are both over 5 years old. He did well in with the mares and their folas but the mares were there to protect the babies from hard play.