With only working with them a few days it sounds like you are moving quite quickly.
How long have they been together? Are you doing 24/7 bonding?
Chasing and mounting is not always bad, is it “i want to mount you” chasing with ears forward? Or aggressive chasing? Mounting is fine as long as it doesn’t lead to a fight.
The rule of thumb is that they should be have 48 hours of no fighting, no aggression and lots of positive behaviors (cuddling, sharing food, sharing litter box) in neutral space, then you can move them to semi-permanent and repeat, then to their final home (cleaned and deodorized as much as possible). If you don’t have semi-neutral space to work with, you can keep them in neutral a bit longer just to make sure they are really really bonded.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.