Why do you say that PPP is a Democratic pollster? Although they certainly had a Democratic lean several years ago, they've recently been close to the mean or slightly R-positive. 538 has them at R+0.2.
(Hmm, is R-positive a Vulcan blood type?)
When doing non-partisan polling, I wouldn't. But if Mitchell Research is supposedly a Republican pollster (presumably) due to their private clients, then PPP is a Democratic pollster because their private clients are almost exclusively Democrats and Democratic-leaning advocacy groups.
Don't people normally put a PPP (D) anyways?
btw PPP polls have been fairly generous to Trump among serious pollsters.
No - perhaps unless they are conducting a poll for a Democratic-leaning advocacy group or candidate.