1) If you feel like breeding for a better meditite, go for it (the move relearner is available after you obtain the poison badge and railnet becomes accessible - just make sure to have rock smash on a mon).
2) Heart scales are hard to come by early, so using it for giga drain on roselia is kinda meh since you need a bit to have access to a shiny stone and roselia will kinda be irrelevant up to then (as a non-fully evolved it will get outsped quite a bit and being frail as it is it won't provide with much in most boss-fight situations). If you don't want to go through the whole process of re-training the roselia, then I suggest replacing it as it doesn't learn any good stab moves by level up, until too late. (Btw, reverse candies don't cost too much, I think like 50 pokedollars or something each?).
3) You can use cottonee later, as it also evolves through a sun stone and I'm not sure if you have access to that yet, but in case you do, go for it. Leveling up it has a pretty good movepool, which actually covers for the roselia slot and adds the fairy stab that you can make use of in your team (even if it's at level 50), as well as in conjuction with prankster, it gets really good support moves (e.g. leech seed, memento, tailwind etc).
4) Yeah that Zangoose isn't looking too hot in general and even though it is immune to poison, which could help in the next gym, it doesn't actually do much. You can get a gothita in the beryl library, which can provide some help with the upcoming poison gym battle and also if you explore the cave west from the beryl gym (after you pick corey's ring - hidden gym puzzle) you can get his nidorina, which learns earth power after evolving on 43 (I think?).
5) Visit the rooftop on onyx ward. There's a pichu there you can catch (with natural good ivs and nasty plot), which you can evolve into alolan raichu in apophyll beach, which can be super useful throughout your gameplay and the next gym battle.