I use Puncturesafe which is a similar product. This is safe for use in a tubed tyre however you might have trouble balancing them and you will need new tubes when you replace them.
I put it in my first set of tyres but when I first got them replaced at MWS (who supply the Blockleys) they advised me not to bother in future - which I haven't

I wouldn't run the back tyre without a sealant.