RH thumb placement during scales

Greetings

Having carefully observed my own playing during practice sessions over the past few decades now, I can say definitively that you will never achieve fast scales by practicing them slowly. While slow practice is good when initially warming up, If you want to play fast you must train your fingers to move fast, even if it's only one at a time, then two, etc. on repeated notes before progressing to full scales. As someone else suggested, one does not learn to run by walking faster.

As for thumb placement, I suggest that you watch any number of guitarists on youtube, both classical and flamenco, and attempt to emulate one of your choice by practicing in front of a mirror. I am particularly fond of Adam del Monte. Also, Francisco Antonio (estiloflamenco.com) explains in detail the exact movement of the fingers during picado.
Practicing the reverse, or outward movement of the fingers as in rasguado, will also benefit your technique. Stepan Rak, who had remarkably fast scales in his younger days, used to recommend playing scales in this manner, a sort of single-string rasguado. I like to alternate this with my normal picado practice.

Hope some of this helps and remember to take lots of breaks to prevent buildup of tension.

Almost forgot to mention: consider practicing three-finger (a-m-i) scales as well.

Cheers

Ernest