I personally don't see what harm it could do. I was taking CS and Senna and psyllum husk before and while it worked to create lots of BMs, it didn't work like the P&B shake. I just kept seeing a lot of fluffy little Psyllium husk balls coming out.
If I were you, I'd get used to the P&B only before you start adding stuff.
As an aside, I have been cautioned that over-use of Senna can make your bowels lazy as it is a powerful contracting sort of drug. It gave me the cramps, just a little bit.
I also now eat a lot of yogurt, acidophilos pills and digestive enzymes after a P&B BM because mine come out violent green sometimes and I think (not sure) that this is a sign of lack of probiotics.