Unfortunately I have no insight on the ospho... never used it, but maybe someone else can chip in on that. The Master Series Silver stuff I used was really, really good, but it is certainly not the only product you can use!
One of the strengths of the MSS stuff is that it functions as a fully sealed surface on its own (so, not porous). If the ospho leaves a similar finish, then yes you should be fine to mix the ceramic spheres into your existing can of epoxy primer and have at it directly on top. If not, then probably what I would do is put down a layer of straight epoxy primer first (to seal the surface), then mix the spheres into the remaining epoxy to put down another couple coats on top of that.
Regardless of what products you end up going with, the beauty of those ceramic spheres is that you can use them in literally any paint. So whatever you put them in is going to be totally fine, as far as the spheres are concerned. Really what you need to worry about is making sure the paint you use is going to hold up over time and not invite rust - probably epoxy primer is fine for that as long as you have good adhesion.
When I did my car, I stripped probably about 75% of the paint before I got tired of it. I wire wheeled all the loose flaky stuff off and scuffed everything real good with a scuff pad/wheel, then wiped down with wax/grease remover to get the dust out of there. That's doing it the right way - I am real paranoid about these things as I have had a lot of trouble getting paint to stick to anything in the past. Well, it turns out that the MSS will adhere to literally anything except gobs of grease, so I totally could have just slapped it over any and all surfaces with probably minimal to no prep. And in the future on other cars, I will do exactly that.
For your chosen products, you will need to consult the manufacturer/instructions to see what's recommended and what you can get away with as far as surface prep goes. If I were you, I'd err on the side of caution, as you do not want that nice paint delaminating and peeling up under the carpet where you can't see it. Good way to get rust starting without knowing about it. Of course, in CA it probably doesn't matter! But those spheres will be most effective if they are actually attached to a surface, probably