I am the new proud owner of a G19, and an HK USP .45 - and was wondering about cleaning materials. I know some plastics can be weakened or even ruined by petrolium products... will cleaning solvants damage the plastic frame on my Glock or HK?

Acetone WILL harm the plastic on a Glock; and, I'm not really sure; but carbon tetrachloride might.  

Here's what you CAN safely use on your new polymer frame pistols:  For degreasing prior to cleaning, (1) Birchwood Casey, 'Gun Scrubber' solvent/degreaser.  (2)  Any NON-chlorinated brake cleaner solution.  (Much cheaper than, 'Gun Scrubber' and does exactly the same thing.)  

(1) For general cleaning and removal of:  powder residue, leading, copper, primer fouling, etc. try, 'Ballistol - Sportsman's Oil'.  It has the advantage of being largely non-toxic and non-carcinogenic.  This stuff is from Germany, available on the internet, and works extremely well.  It actually makes the polymer on my Glock frames shine!  It will get the bore spotless if you let it soak in for 5 minutes and, then, wrap a cotton swab around a, 'Tynex' (nylon) bristle brush and scrub the bore - all the way forward and all the way back, about, 15 or 20 times.  Follow with a clean patch(es) and repeat one more time - if necessary.  

(2)  On really heavy fouling I'll use either a patch from a, 'Lead Wipe' rag or, 'IOSSO Bore Cleaning Paste' on the first patch and, then, follow up with Ballistol.  (Both patches wrapped around a nylon brush because the old, 'patch and jag' method, while suitable for rifles, is the really HARD way to clean a pistol!)    

(3)  Then there are the other usual gun cleaners:  BreakFree CLP, MPC's FP-10 CLP, Eezox, Clenzoil, Hoppes, Kroil, etc. which are, all, good to use on any polymer parts.  Sometimes you have to use whatever is available; and, at one time or another, I've used all of the above on my own firearms.  Some require more, 'elbow grease' or patience than others; they, all, work; and, in a pinch, I've also used just plain IOSSO gun oil - it works, too, and just about as well as any of the above.  I like the IOSSO oil because it's very fine and very pure.  I, also, use it on my power tools, shears, electric razors, and the wife's sewing machine.  h-coat
One precaution I ALWAYS take whenever I'm cleaning guns it to wear, 'rubber' gloves and make sure I have adequate ventilation.  I buy boxes of disposable latex gloves at the drug store; but a lot of guys I know go one step farther than this and get chemical resistant, 'Nytrile' gloves from the hardware store.  

All of the above cleaners and solutions can be found with a simple, 'key word' Google search.  Good luck to you with your new pistols.  H&K makes fine guns; and the Model G-19, just, might be the very best of all the Glocks!    

Use CLP.

You've got both the long and short of it.