Gold will be an issue until you get a solid team of high lvl girls going, where you don't need to upgrade the main squad anymore.
When times are tough on gold, just reduce your costs - for example if you upgrade a girl with 10 manyus, upgrade her only with 1, etc.

We all struggled with gold here and there, the more girls you evolve/bloom and upgrade the worse it will be. I remember very well burning million upon million of gold coins when I mass evolved/bloomed girls. Heck I had to stop evolving/blooming for almost a month, doing just small upgrades and keep playing maps. When the 2x gold bonus came, I ran the Mori Mori 100 stamina gold map over and over again. I burst upon few dozens of Stamina potions and just kept running that one.
Never forced it to much just tried to get a million or so to last me for a while, but I distinctily remember someone, maotd or volarmis or someone who went full ham mode and earned several milions, lasting him for quite a while.

So the simple answer is > Prioritize, tighter your belt and keep saving up for the priority girls.

As for the special missions, well yeah they are not supposed to be super easy, since you get a D-medal for it. Still a full 4 teams of bloomed 5* should be good enough.

As for the ticket - if you have a setup in mind go with a girl that compliments it, otherwise go for who you like the looks and skill off.
And yes - you can have both in the same team. Heck this game allows you to build a team of the same one girl. I remember someone doing something like that with ...Red Ginger ? not sure, but I remember seeing a pick of a team made out of the same 6*. The other teams were the usual mix but one was made of singular girl. So absolutely its possible