I have a Viking Designer series cooktop with the Dacor downdraft. While I had the units on site when the counters were templated it was up to me (I was the GC) to confirm all the calculations to determine the cutout size. Actually, I did that before purchasing the appliances to ensure they would fit. While I was totally disgusted by the dishonesty demonstrated by Dacor (long story) and would never buy another Dacor product, the units do fit together.

The flame on the back burners is pulled by the downdraft - a lot. It interferes with cooking and the downdraft is scorched. To mitigate those problems, I would recommend getting the tallest downdraft on the market and give up the cabinet space.

In my last house I had a Dacor cooktop and the Dacor downdraft vent, so I can't speak to mixing brands, but I will reiterate what judeny said. The downdraft pulls the flame from the burners and disrupts the cooking. If you have the fan on high enough to be effective as a vent then it will even pull and distort the flame on the front burners, and it's incredibly noisy. I had mine installed in 2003. Sold the house in 2010. I woudn't buy a downdraft again.

judenyand mlweaving_Marji thank you both for your input.

judenyand we haven't ordered anything yet and we will be installing the cabinets ourselves so are making sure at this point that everything will fit in the cabinets. The Dacor directions have the downdraft inserted into the cutout for the cook-top then wiggled into it's slot. This is our problem at the moment. We had been hoping to install the downdraft behind the cabinet (we have a T shaped island so it would just be filler strips in this area). If the top could come off of the downdraft, we could insert it into the top of the counter and into the bottom part of the downdraft. My husband's interpretation of the directions is that this can't be done (hence the wiggling into the downdraft space). If anyone had installed a Dacor downdraft we'd love to hear how you did it. Without the physical unit here we have no idea exactly what can be done.

We will have an induction cook-top so we shouldn't have any problems with the flame and the downdraft. mlweaving_Marji our last Dacor downdraft had the fan on the roof - this unit's fan will be in the basement (inline) so the downdraft shouldn't generate a lot of noise in the kitchen.

The biggest reason that we are going with the Dacor is that we have black counters and Dacor makes a black top downdraft. Our appliances are black too. We also liked our last Dacor downdraft except that it was too short - the new ones are higher. We don't have the option of using a hood - he have to exhaust down.

I haven't looked at current specs or installation instructions but I did my downdraft install and duct design.

First, your inline blower set off sirens in my head. Please have Dacor mail you their specs and confirm that their current spec permits the inline blower. For me it was 6 weeks from receiving specs from them until I purchased the unit and installed the ILB. They changed specs without notice (I found out accidentally) and then it took a lot of calling and arguing to get them replace the ILB with a remote blower.

When I did my install, the cutout for the dd adjoined (and was narrower than) the ct cutout. The dd does not come apart. It was lowered straight down through the counter cutout and screwed to the bottom of the cabinet. Mine is in a standard depth cabinet in an island. I ducted through the back of the cabinet and down between the floor joists and then a run to outside to the remote blower. I then installed cabinets behind the island on a platform raised 9". In the 9" space is the ductwork curving into the floor, the gas shutoff valve, the junction box for the oven (undercounter next to the cooktop). All wrapped with house matching panels.

So the counter must be installed first.
You need to be able to access the front of the unit to anchor it to the cabinet floor.
You have to plug it in to the outlet. (My outlet is installed in the far back floor of the cabinet.)
You have to be able to access the side that you are going to be attaching to the ducts; I vented through the back but my unit had more than one option.

You need the right tools. I had a couple of very large drill bits and what I believe was an electric coping saw to cut the cabinet backs as necessary.

You need to be able to access it for servicing. My island back cabinets with house matching panels fit together like a puzzle but removing 4 screws lets you move a cabinet (with attached panels) to expose the the duct attachment. (Removing one screw from a panel for the gas shutoff, another screw from another panel for the oven junction box., I have 18" drawers (standard is 21") under the cooktop that all come out to allow access to the unit.

Just an update: we did install the Dacor downdraft with the 36" Jenn-air induction cook-top and all fit in fine. Since we are all electric, I have no problems with the downdraft pulling flames from under the pots. With the inline blower in the basement this venting solution is really much quieter than others - I really like it!