To answer when
clen should be taken, I'd say probably not in the same cycle as deca!
Deca is good at building mass, and
clen is used for shedding the fat - eg, one is for bulking, the other for cutting.
I know some people don't, but the general rule is that as Deca shuts down your natural
test levels bigtime, so you'd normally use
test as a base, and stack it with Deca. Running Deca only will kill Mr Winky and you can forget him looking skywards for some time (Deca Dick)
Check out the stickies at the top of this forum and look at the example first cycles. I think you'll get better advice here than from what your friends tell you.
For the record, I know people who've used bad combinations of gear doubled up with bad diet and bad training. Even after 12 weeks, it was very hard to tell they'd been on.