At last, a question arises on which I can possibly help! Practice stalling is done in gliders at any height, no restriction at all, in the UK, and I daresay the same applies to the rest of the world.
Two reasons for not needing a lower limit. Gliders when in soaring flight are usually flying only a bit above stalling speed, and when you reach the core of the thermal, you crank it in big time! so not only at a speed only say 5 knots above stall, but at a considerable angle of bank as well. Which means we experience stalls and the approach to the stall frequently. I now have to fly with a safety pilot, due to age and other limitations, and I am always making him nervous by tight turning with a touch of prestall buffet.
So we practice stalls and stall recovery a lot in real time. It is a lot simpler and a lot less dramatic than the routine in a power plane, like a Cessna l52 for example, when the nose being heavy, it drops more suddenly, and the power used to aid recovery is an additional hassle.
In a properly trimmed glider, simply moving the stick forward a trifle nearly always will restore normal flight in a jiffy, with no loss of height. So no big deal, and we practice it a lot. Also I used to tell my students that a good landing is a stall near the ground. Frequent stalls, without drama, not so easy to practice in power.
Spins, now, that's something else. A good way to loose height if you need to descend. But NOT to be practiced less than 800 feet in a docile and reliable training glider like the K13. In glass, particularly certain types, I prefer a minimum start height of at least 1,500 feet. With a 3 turn spin you will lose probably a minimum of 500 feet before recovery is complete. Delightful! However, you cite 50 or 100 feet of height lost in a recovery;
is this where you bottomed out? I think it would be rather more than that...but of course you are trying to maintain a required altitude for separation from other aircraft, possibly in IFR. Gliders habitually fly close together, in VFR, and without ATC. hooray!