I highly encourage you plumb your shower so you can run anywhere from one to all three heads at the same time. Figure out the two heads that will be used most often...for example, you using the handheld and your hubby the fixed. I'd have one valve control the handheld for you, the second valve control the regular shower head for your husband. Then put a diverter on either valve to control the rainhead. Or, get one of the A/B/C diverters that can send water from a single source to either A, B, or C, or any combination of the three. Here's an example. It really will help with flow control. If your supply is 1/2", then just use a half-inch valve. Do realize that with some of the diverter valves it's assumed that there is going to be significant flow required, so they might not be available in a half-inch size. Only larger. Pressure balanced valves feed rainheads just fine. I've neither seen nor heard of a problem. Before doing any plumbing, I'd see what flow volume you're getting out of the half-inch pipe, you'll need to know if it can supply three outlets at once. Example, your three heads are each 2.5gpm, you'll need 7.5gpm out of that pipe to run all three at once. If it can, then you can do as you want. If it can't, then look into a 3x diverter. Or multiple valves, one for each head. Or something in between. Your choice. If you go with a single valve, be aware that the valve may have a volume pass-through restriction. Make sure the valve can pass through the 7.5 gpm that would be required in the example above. Mongo

Cost of doing 1" vs. 3/4" while you are replacing it anyway will not be much higher. I have 1.25" for the main line coming from the meter, then down to 3/4" partway through the crawlspace and running to all of the destinations. The actual connection to my 120k BTU furnace is 1/2", as are the ones to my (old fashioned tank )water heaters. While the walls were out, I stubbed out gas to the range location, even though I'm installing induction. Cost little, and I like to keep options open.

Most ppl erroneously believe their line starts at the meter, but that is not correct. When we install a new gas service the gas company will inform us where our final connection to the "gas main" will be located. Normally the property owner is then responsible for installing a line from the structure to within a specified distance from the main, usually on the utility easement on the front of the property, about 4' to 5' from the municipal main where they will install a "Curb stop valve". The curb stop is the point of demarcation between the gas company line and the homeowners line. The gas company will also inform us where the meter is to be located. It may be near the street, adjacent to the exterior of the building or is may be inside the structure, such as in a basement or utility area. An exception to that may occur if the gas companies line has a higher pressure and they install a pressure reducing valve at the meter, in which case the homeowners line may begin at the meter. For the purpose of sizing line we must measure the "TDL" (Total developed length) of the line from the designated point of demarcation to the furthest gas appliance or demand. TDL is determined by adding the actual physical length of the line plus the "Fitting insertion loss" of each fitting on the line. To find the fitting insertion loss we have to consult a "fitting insertion loss table" for the specific type of pipe being used. The table will list each type of fitting and express the loss as an equivalent length of straight pipe, by example it might say a 45el has a fitting loss of 0.4ft and a 90el might be 0.7ft. Under most codes they will allow us to compute TDL by multiplying the actual physical length by 1.20%. Example, if you have a total of 100ft from the point of demarcation to the furthest demand the line length would be 100' x 1.20% = 120ft. We would then compute the total load in cf/hr and check the table for the load and TDL to determine the correct line size. In regards to whether you can install a union in your crawl space, you will really have to check with your inspector. Normally if the crawl space is open to atmosphere there would be no problem, but, if your crawlspace has a skirting around the house with an access door that would normally be considered a concealed location. The concern is that you could have a gas leak and the gas would collect under the house unnoticed and potentially create an explosive hazard.

1/2" steel pipe would be fine if you had a straight shot. Once you add in fittings and elbows, however, you will be pushing the limit on pipe capacity. I would size up. The yellow flex line is generally fine as long as it is properly bonded and grounded - check your local codes to be certain.