Before I go further,m I remind you that the devil is always in the details. So while the general may be OK, your specific application may not be.

To start with derailleur indexing lives in the lever. The lever pulls a set amount of cable, and the RD translates that to the correct horizontal movement to match the sprocket spacing. So, the key it derailleur interchange is that it and the lever speak the same language, meaning that they use the same cable movement/RD movement ratios.

Companies use these same ratios for a while, then may change, so odds are that if you use the same brand RD form the same generation (ie. any 10s derailleur made within a few years as the levers) it will properly index with whatever the levers dictate.

So, it's not a question of using or not using some gears, as much as matching the lever, which matches the cassette.

There are also other more minor factors like whether the idler cage width being enough for your chain, but those aren't usually issues.

Since you have a 7s system, I suggest you stay as close as you can with a 7s or 8s derailleur of the era, which will most likely be less expensive than a more modern one.