I would just mix the perlite in with the soil . I have used mg with added fertilizer SOIL before it was fine . so if the perlite is the same you are doing just fine . mix it in no problem . keep in mind its hard to flush time released fertilizer . at least 2 week flush.
Nothing like giving your plants a double dose of free nutes...then wondering for days after 'how you burnt them to death' ....WASH ..wash really well in a bucket ...rinse well, air dry