Below slab plumbing advise

[NateR]

Hello all. I'm starting planning a new home was wondering if anyone would give me some feedback on the below slab plumbing outlined in the attached image. I don't think I'll try this myself but I would like to be more knowledgeable in general before I talk to a few plumbers as I'm in a new area and haven't gotten any strong recommendations.

Questions:

• In general is this feasible? Any major issues?
• Can one adapt from 2" to 4" (from shower trap in lower left corner marked - Sh)to start the 4" run of pipe? Code that pipe size has to go up when multiple drains of the same size come together?
• Venting - can a vent be run from the wall in between the tub and shower to just one of those lines and work for both or should it be attached to both? If both what type of fitting to branch to both?
• Is venting each fixture overkill or worth it given new construction? Guess I was thinking one vent for shower and tub and one for toilet and sink connecting all three above.
• Does the utility room drain on the far right need venting?
• I kept the toilet pipe size at 3 as I read somewhere that might be better given low flow quick flushing toilets these days?

Sorry, that's a lot of questions but I've been stumped thinking about this as many of the diagrams I've found seem to run toilet -> other fixtures --> onto sewer exit.

Thanks

 

[Terry]

Flat venting below grade isn't done unless it's washed by wet venting.
The tub can wet vent the shower, meaning one vent there. I would turn the combo up in the wall so that the vent portion is vertical. The trap arm can be on the horizontal.
The toilet vent should be vertical. You may want to come to the toilet from a direction that allows that.
The floor drain will need a vent.