Are there downsides to cross-top brake levers? Compatible with all mechanical-hydraulic disc brakes?
I am a maker of custom bikes for kids and small adults. I am a heavy user of the interrupter-style levers, even for main brake levers. The main reason that I use them is that they are easy to adapt to a custom bracket which fits smaller-diameter handlebars and smaller hands. Another great benefit, they are some of the lightest levers available, period, at as little as 45 grams each in stock configuration. I am familiar with both Tektro and various clones.
To answer your questions,

It is not likely to decrease the braking efficiency of the main levers. I doubt you will notice a difference. Technically there could be some more friction from the cable flowing through the lever, but the lever itself should have less compression than brake housing. To minimize friction, consider using lined end caps such as those available from Jagwire here: https://jagwire.com/products/small-parts/lined-end-caps
These types of levers are &quot;road pull&quot;, so they should be compatible with your Hy/RD road calipers
Tektro is a safe option but the various clones available on the market seem almost identical in function and even in appearance. Do be advised that many of them come in strictly left-hand and right-hand versions, according to the shape of the lever itself. This could be important if you plan to put the lever in an odd location, or in a &quot;reversed&quot; configuration.