We do not recommend installing TireJect with any other tire sealant product.
Many Off-Road customers have reported that adding TireJect over top of Slime (or other thick gel-based sealants) and reported success sealing the tire. However, we do not recommend doing this for a few reasons. We cannot guarantee a permanent seal given that we do not know the long term effects of mixing the Slime (or other) tire sealants and TireJect. Slime (or other gel-based sealants) can cause tire balance issues and cause pre-mature tire dry rot and we do not want you to associate the negative outcomes of Slime with our product.
Our official recommendation is to remove the Slime by removing the tire. Use a putty knife to scrape out the Slime and then soap/water and a cloth to wipe out as much as you can then finally rinse with a hose. Be prepared Slime is a huge mess to deal with. As a comparison TireJect is 100% water soluble and easy to clean up with a bucket of cold water and a cloth.