You're in the right section. No worries. Posters here are just going to put your safety first, and you should too. From the experience you listed it sounds like you have little to no tolerance. This means that you don't want to combine CNS depressants at all as an overdose becomes very likely unless the individual is tolerant to both substances being taken. Even then, it's not what can exactly be termed safe.
As has been said, try one thing at a time and be sure to space them out adequately. Also avoid the methadone all together. Period. Even at a very low dose, it's going to be way too strong and you'll feel sick as opposed to good.
Welcome, regards, and stay safe.
-bronson