The answer is 'it depends'. You don't say where you're going, but if you take Chef Mickey's as an example - a couple months ago we were only able to get 2 reservations for 4 (for our total party of 8). We got to the podium a few minutes early and requested to sit together. They told us they'd try to get us together or near each other. That particular reservation, they did work it out that we sat together. So if you can only do 2 reservations with 2 members of your party, ask at check-in.
A quirk of the ADR booking system is that it seems to not like odd numbers. So I'd look for a table for 8 and book it if available. There's no penalty for booking for 8 and showing up with 7, but if you want, you can go back after you book it and try to modify down to 7.
That's less likely to work for you. It's way more difficult for the restaurant to add 1 than to reduce 1. 7 could be a different table type from 6 - so I'm not sure I would count on doing that. Never say never, but booking for 6 and hoping to sit 7 might not work.
You will be better off going with 8 than 6. You might be seated but you can be seated much later. Unless a prepaid meal no charge for being seated with one less and most likely you will be seated at a table that seats 8 with your party of 7.
If it is a possibility just book for 8. Unless it is pre-pay it would have no impact on your ADR to show up short one or two people. At least they would have a table set aside for your party, otherwise you might be out of luck.
