Honestly just be patient and they will figure it out.
In my experience it is not helpful to try to sway a particular outcome or hierarchy in bonding (I’ve tried… it doesn’t really seem to speed things up, and can seem to slow progress). Even if you think Thumper will be dominant, that may not be the case, so try not to assume anything at this point.
If you trigger grooming before they trust each other I think it can cause confusion in their communication. Once they all trust each other the grooming will come naturally. Juice or banana on the forehead can trigger grooming, but whenever I’ve tried it too early in the process it hasn’t really helped. If you get to the point where everything is great except that (they are snuggling and sharing food, litter box, etc), then I think banana on the head can help.
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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.