Firstly, experimenting is great and can lead to all sorts of discoveries and improve your understanding of the colouring process.
However, I’m not sure what the legal position is where you live OP, but in the UK and most of Europe, if you mix developers from a different brand to the tint, you won’t be covered by insurance so apart from experimenting on yourself or on blocks, I wouldn’t do it.
All the insurers include a clause ‘follow manufacturers instructions’. All manufacturers state in their instructions to use the matching developer so if you don’t and the client suffers a reaction, you won’t be covered. Disclaimers have no legal standing here to protect the hairdresser and in fact, can be used by the plaintiff to prove that you knew doing X was potentially dodgy, so we always advise not to use or rely on them.
Obviously, things might work differently in your part of the world so probably best to check.
Also, friends and family can end up being the worst clients when things go wrong, so I’d advise to always keep it professional.