As the owner of a Pilatus B4AF, and with some experience on B4/21/Fox, I feel I can at least give some hints.

I started with some figures on Utility class gliders. These often have max G ratings of +5.3 and -2.65 in my experience, although this is at Va speed. At Vne they often are limited to +4 and -2,
which is to take flying through strong thermals into account as far as I know.

The G-limits on gliders and allowed figures are two different things. Especially the high-performance gliders pick up speed fast.

I found that a truely Aerobatic-class glider (which often have +7 and -5 rating) is also suitable to learn figures on. In other words: if you screw up badly you will still not easily damage the
glider.
Think about kicking the rudder too late in a stall-turn and then ending up in an unintended tail-slide.

Also, the high-performance gliders often have a very laid-back body position for the pilot. In the DG-303 Acro handbook the pilot is warned about this, since G-awareness is lower in a reclined
position. And thus the risk of accidentally overstressing the aircraft.

In my B4 I fly rolls, inverted flight, spins, rolling turns (rolling and turning combined), tailslides, stall-turns and cubans (both forward and reverse).
I often have roughly +5 and -2 on the G-meter after a practice run. The +5 is during stall-turn pulls (to get a nice line) and the -2 is achieved during the pushing in rolling turns.
I would say for the first inverted flight and rolling (see the German Prufungsprogramm) you wouldn't go beyond +5 and -1.5 (achieved when pushing out your speed in a Split-S maneuver).

Long story short: I would recommend you learn to fly aerobatics on a Aerobatic-class two-seater. Then, if you become addicted, get yourself a B4/SZD-59/ASK-21.