I have never tried Slime or anything like it so I cannot comment on this matter. I can comment on new tires, as mentioned above. My wifey bought a stroller she uses for her dog while she walks the neighborhood. The tires on that thing are horrid. I have even purchased new tires at bike shops and they are the exact same not wanting to hold air by slipping off the rims or on several occasions blowing out the tubes.My tractors all have tubes in their tires because tubeless tires almost universally leaked at some point in their lives (maybe the Massey's fronts don't; I've never had trouble with them, so I've never needed to check). If they leak, the tubes get old-fashioned patches, or they get replaced.
I had (actually, I still have) a tubeless front tire on my 4000 that would be fine all day if you put 40psi in it when you started, but the little ******* would roll off the rim if you only put 39psi in it. Let me tell you what, carry a farm jack, pancake air compressor, air hose, and a rubber mallet to the far end of a hay field (and back) in July just once and that will teach you to make sure you got the full 40psi in that bloody tire the next time.
My dad tried Slime in some of his mower tires, but I don't recall him ever having good luck with that. I think some of those buggers just had an aversion to air on the inside--some of them would leak with brand new tires. Putting tubes in them was the only way to stop some of the leaks.
As noted above, tire shops do not like sealants like fix-a-flat or Slime, so it's best not to use them on tires that you can't fix yourself.
The wire beads on the tires don't even come together at the separation point. I thought it was the rims at first because the rims had exposed TIG weld beads which made the tires not even seal so we had to use tubes. I even ground down the rims with a Dremel and they still blew out. Cheap Chinee Sheite.