Hey @CMaxi,
yes it's possible and shouldn't be that big of a job. This mod just chance one resource: the jig of the play guitar action. The jig is an invisible object that's placed beneath your Sims to reserve some free space. You can adjust the size of that jig with S3PE quite esaly (open file with S3PE > right click on FTPT resource > Footsie > edit the sizes).
You can repeat that process for any jig in the game, including that for the pillow fight.
To find a jig, you can use S3OC to search for Objects (open S3OC > Tools > Search > type in: jig > clone what you need and edit the package)

I might some more jig edits myself, but now you have the quite easy way to produce some jig edits yourself

Yes, it was some invisible jigs. Apparently, if a dog is chewing on a toy it can somehow be interrupted but not clean up the jig. None of the tricks that I found online for finding the jigs would actually show them, but I was able to just sort of guess at the correct one using NRaas MC to list all items on the lot and choose some to delete, and that seemed to clear stuff up.

This and the bass one are amazing. Is there anyway you could minimize the size of the fishing jig so it won't require so much space from a houseboat?

Shouldn't be that hard to achieve: I created a short "how to" for creating a jig-edit just 2 posts above this one

Thanks, no more walking outside to play the guitar!

Christ on a bike, thank you! My house isn't even that cluttered with plenty open space, yet they still hurry out the door into the snow in their underwear to learn guitar. Now my sims stand in their kitchen playing guitar instead of freezing to death. Thanks a bunch!

Exactly the problem I had before
If you liked this one, than you'll really like my other mods, that do the same, but on a bigger scale
Sims need less Space
Pets need less Space