Regarding weights: I started using dumbbells about 4 years ago, using an ancient pair of just 2.5 kg that someone gave to me. Since then I've acquired 5 kg, 8 kg and 12 kg, and I still use all of them, depending on the workout. If I'd known that I'd get so serious about weights I'd have boughta good set of adjustable dumbbells.
I'm always very careful when I start using a higher weight and I've never injured myself. I try to challenge myself without going overboard, and I have become very attuned to my body.
As for Caroline Girvan, I would suggest doing her Beginners workout several times - as long as it takes to get familiar with the various moves and feel comfortable.
I always watch her YouTube workouts the night before I do them so I know what to expect. I also listen to her very detailed explanations and demonstrations at the start of each workouts, particularly for compound movements. Typically I use about half the weights she's using.
When I started out I would modify a lot, e.g. just do basic Bulgarian rather than elevated lunges, dispense with yoga blocks or resistance bands that make movements more challenging, do hip thrusts on the floor rather than with shoulders on sofa, et cetera. If I get tired I do alternate pressups or raises. When she does all kinds of burpee and pushup variations I just do the basic ones.
It's me against me and I'm just happy that I can now do 15 straightforward pushups, which i never thought possible when I started out.