Grease on under-sink shutoff?

To start off, I live in Maryland (for potential code purposes). I am looking to have my home converted to natural gas since my power company offers some incentives and I prefer gas for some of my appliances. I am in the process of checking to see if my house has a capped gas line from prior residents (home was refurbished recently with all new electric... everything). It seems as if my street has a gas line, but I haven't called the energy company to fully check yet. My question before I really get to deep into everything is installation of pipes. Assuming that when they redid the house they removed or any prior lines (if any), I would need new lines installed or at least lines to where I want the gas. Being that I am not a licensed/insured plumber or a professional, I would have to have some help in this area. But trying to cut time and cost from a plumbers equation has led me to question if I could pre-install the lines and have the plumber conduct the tests on them. I feel it would be the same as say, the lines already being there and having a plumber come out and inspect and ensure everything is good and sealed to have the gas turned on. Please correct me if I am wrong, I am just like most of us trying to save a buck or two by cutting out the middle man some. I would of course still have the plumber inspect and pressure test everything, I would just lay the ground work.

I am redoing my old 1/2" underground single home run copper pipe with Pex. I am going to be using a manilfold in the garage and run branches to each wet area. Kitchen, master bathroom, guest bathroom, laundry, outside faucets. I was planning on running 1/2" to everything except the bathrooms. There I would run 3/4" pex to the bathrooms and tee off to 1/2" to the sinks and toilets while continuing to the shower with 3/4". I am wanting to minimize water pressure drop to the shower when sink or toilet is used. Is this over kill? Does anyone have 1/2" the bathrooms and have water pressure drops like i'm describing? skeeter