Everything's going according to plan. Check out Youtuber Sariel's review on your set HERE.
At about 26mins in, he addresses your concern. He notes the looseness of the feet when fully deployed, and says there are 4 pads stored in a compartment over the front axle. Just build on.
There was a Technic sub-theme called Universal that was available mostly in the 1980's. The sets did come with instructions to build as many as 4 different models, but each model rarely used all the parts. The models were more of a demonstration of how to use the parts in interesting ways.
You can browse the sets in the Technic Universal theme here:
The presentation that senior Lego designer Jamie Berard held at Brickcon 2006 and that spawned the entire legal vs illegal discussion can be found here:
The particular case you ask for is discussed in slides 9 and 10:
According to LEGO, this type of connection is illegal, since the pin is stressed when connected to an anti-stud and eventually deforms.
People's MOCs do not follow same policy as TLG does. So you may encounter illegal connections from time to time. Pins are also cheap and usually owned in high numbers, so nobody's really bothered if one gets deformed.
There are numerous solutions over internet for attaching SG90 servo with LEGO. However I remember this particular video, which fit your need to attach/control Technic beams/liftarms. It also features required parts for this build at 7:48.
There is an alternative app called BrickController 2. It is available for both Android as well as iOS. This application is actively developed and its maintainer can be reached on Eurobricks forums. You can also report and discuss any issues there.
The application is universal compared with solution offered by LEGO since one device allows you to control ...
The EV3 expansion set 45560 comes to mind: https://education.lego.com/en-us/products/lego-mindstorms-education-ev3-expansion-set-by-lego-education/45560
Although intended as a extension to the educational EV3 set, it's mainly technic elements.