I want to re-plumb my house using a manabloc system and I want to use 3/8 supply lines for most of my fixtures. The stub outs that I can use are 3/8 pex to 1/2 copper. So with that said and without me tearing into walls am I able to just use the copper pipe adapters within 2-3 of the fixture to hook up my new 3/8 lines without losing pressure or causing any type of problems?

You said, "am I able to just use the copper pipe adapters within 2-3 of the fixture to hook up my new 3/8 lines"...? I am unclear here...are you asking if you can pipe 3/8" PEX to more than 1 fixture without issues? If so, I say, NO WAY!!

Volume is the issue here...not pressure! I would never run 3/8" pex to any more than 1 fixture , and even then, only to bathroom faucets and toilets. When it comes to outside faucets (sillcocks), washing machines, dishwashers, kitchen sinks, tub and tub/shower valves you will need, at a minimum, 1/2" PEX to each fixture. Here, if you pipe more than one fixture with 3/8" PEX, you will see a decrease in volume when someone uses one of the fixtures. Further, if feeding less than 1/2" to the fixtures I mentioned, you would also see a reduced volume/pressure of water when someone uses another fixture in the home. That would be a real shame when you are showering, trying to water the lawn, or trying to fill a lobster pot with water, etc., right?

Anyway, glad to discuss more if you want...

Mark