How much(injectible test) does it normally take to shut down the body's natural production of test?

I've been using low doses of test cyp and deca.

It's my second cycle, I used pct last time. Kept majority of my gains and never lost sex drive.

My second cycle consists of 250mgs per week of test cyp and 250mgs per week of deca durabolan.

Is that low of a dosage enough to shut my natural production, and I'm on my 3rd week.
Should I snag some nolvadex?
Another question about nolva(I usually use clomid). I've seen several different brands of nolva and I've seen it on steroid(real steroid) sales webpages also.
Is it the same compound. If not, what brand works the best?

Thanks, BDA