We had the opposite issue - the flooring in the room where we have relocated the kitchen is 1" lower than the flooring in the adjoining living room. We didn't realize this until we removed a wall between the new kitchen space and the new living room space. I did not raise the kitchen floor b/c it was too much money to do so (we also had other structural expenses that had priority over the floor). We have a transition piece between the two rooms (the opening is about 4'). It seems to be working well (we aren't back in the space yet). If you went back and changed the height of the floor after you do the kitchen, would you remove the cabinets to change the floor? I wouldn't think you would do that. Then your floor would be a different height under the cabinets than in the rest of the kitchen. I would do it all now or do a transition piece and keep the heights as they are.

#2 - insulated ceiling/air tight (IC/AT) is mandatory in many areas and always recommended if installing in a ceiling that is insulated, or should be. Most low voltage housings are only IC/AT if new construction type. Light shop was correct in recommending new construction. #3,4 - looking at attic for access does not mean that the housings will fit where desired unless they also carefully measured the layout to see what may be in the way of each fixture. The layout is generally done from below and projected up onto the ceiling with exploratory small holes to then locate in the attic. That the electrician found the space issues when starting the project should be foreseeable by him. How the electrician priced the project may reflect his experience. Low voltage new construction housings are generally larger and harder to place than non low voltage new construction housings. The fixture you ordered may be a brand or model he hasn't seen before. For example, Halo has a 3" low voltage fixture made for shallow depth but the housing is huge in length and width and can be quite a surprise if one hasn't used them before. If you don't need the shallow depth it is also a mistake to order that model. #5 - sounds like store does not stock the housings. Buying product that may or may not fit well from stores that don't stock and won't accept a return is risky. Electrician should try to put in the new construction. If you aren't in a mandatory IC/AT area then you can substitute remodel housings where necessary and at least limit the number you must return or eBay. And then there is the issue of why and how you ended up with the number and type of fixtures you are using but I'm sure others will add to the thread about that.

Blocking not required, rarely put in - but it's very nice. You can also use plywood in the key areas instead of drywall. You can also use ply instead of the 2x blocking. On metal studs it's almost impossible to hang wall cabs without sheetmetal on the studs behind the sheetrock. Even that can be done though. 30" cab = 4 screws if possible, 6-8 is overkill and just uglies up the interior. You need at least 2 screws (1top , 1 bottom) into a stud or framing in each cabinet minimum. Systems with hanging rail are a different animal all together. On length , general rule of thumb is 2/3 in the meat holding it up with 1/3 in the cabinet. This obviously won't be possible with metal studs. DO NOT USE DRYWALL SCREWS.............................EVER.

I was framing out an area in my basement for some cabinets (the in stock cheapie HD or Lowes brands). Between the studs, I was considering installing a 2x6 horizontally so I can have more freedom when screwing the cabinets in. I am just wondering what if you do not land on a stud in the location you need to screw into. Questions... 1)Does this make sense? 2)Being it is only 1.5" thick dimension lumber, is this thick enough. I never installed cabinet before so have no clue how deep the screws need to go? 3)How many screws do you usually need for a 30" cabinet?