First off, any allies you get, place them on your team and get their rapport to 100, this is going to net you 10 rubies if it's the first time you've gotten them. Afterwards they'll have a "Mastered" marker underneath them, if you have a unit that isn't mastered, it's just rubies that you could have in your pool and if you don't get them before you use them as fodder, you'll have to wait until you pull that particular unit again to get them. As you master more units you'll get other ruby bonuses as well.
To upgrade S to SS I usually just throw C units at them until they're maxed, but starting off you may not have the money to do so, so it may be better to get S units to +4 or so with C fodder and then finish them with B when the gold cost gets higher.
Can take that same strategy for SS units, just move up one tier and use B until +4 and then A until you get to +max. I wouldn't use S to upgrade SS units, however, you'll want to keep any S you get for upgrading your SSS units or turning into SS.
A and below are typically just used for fodder for me, I may do a A->S combine to get the weekly challenge done and get a free premium summon, though.
Starting off, don't combine your max SS units until you're sure that you won't be needing them anymore, a +max SS is going to be more useful than a +0 SSS unit and you might get a pull that isn't useful anyway.
Use your rubies to unlock the second and third consumable slots and fill them with resurrection scrolls, they can get your team through some tough fights. Fill your friends list with anyone who will accept your requests or requests to be your friend, you want to gain friend points to help summon allies and they'll be a major help in later dungeons, you can remove 10 a day so if there are people on the list who are inactive for days at a time, just carry them into a dungeon then remove them afterwards and invite someone else. If they're not active, they're not feeding you friend points, so drop them.
Any other rubies you have, I'd use on Ally summons during the event, some of the outfits look neat, but they're not going to give you very large bonuses and are really expensive. They're mainly there to drain you of rubies and if you're wanting to play for free, then you won't necessarily ever be able to afford them without hindering your progress into the game. You might want to unlock more inventory slots, but that's totally up to you, it will allow you to turn on auto dungeons and leave the game without having to manage inventory as often, but I didn't really start running out of space until chapter 2 when I had to start holding a ton of different essences and jewels.
The main thing is going to be patience, this game is going to give you a lot of "just one more" moments. You'll almost have that unit maxed but you just need one more summon and it's going to really make you want to buy rubies, but if you use them smartly and have a bit of patience, you'll never have to since they keep flowing in fairly frequently by completing daily/weekly quests, login bonuses, world bosses (you should at least attempt them once so you can get the bare minimum rubies awarded, 2 tix are worth a lot less than 2 rubies), labyrinth and arena. Guild battles will also get you rubies, but depending on how active the guild you're in is, that might be kind of hard to get someone to help you with, but again, one attempt will net you something.
The game isn't up, so I can't really picture all those units you've posted, but I know Rar and Eirden are DPS units so they should work well with your warrior main, just make sure you have a priest, preferably one with an AoE heal, but healer Unity will do if she's all you've got. Ranran is a decent S unit healer, if you have her, I'd max her and then use her on your team until you can get a good SS or SSS priest, I had to use her forever and she got me through all of chapter 1 and into a good chunk of chapter 2. The last spot can be used for whatever, a rogue or archer if you've got one in those SS units would probably be best.
At least now I can farm lvl 67 gear
