Nurse Tongus:
Depends on what the rest of your team looks like. There are a lot of areas where you'll need a high skill level healer--the black oni, three kingdom event we got going on the JP side right now is one example. She could also come in handy when you're farming stars and trying to get those annoying "get X bonus...punis (the jelly things you're popping) in a single event.
Tongus works well with the healer butterfly that you get from fusing two other butterfly youkai (the orange and purple butterflies), since they share the same tribe.
Still, you're looking at 500 Y Money each time you fuse her and I think you need to farm 32 of them just to max her skill.
I haven't maxed mine and it's low on my list of things to-do since Y Money is your biggest bottleneck.
Y Money to unlock stages:
As for using Y Money to unlock stages, I'd say that's a definite "no-no". It's not that big a deal, but you're always going to be pressed with the decision to pull more youkai from the gacha OR to raise the max level of your youkai. S rank Youkais take a LOT of money to raise: unlocking levels 40-50 costs 4000 Y Money alone. Same with continuing, unless you're stuck at a boss (or have had a rare youkai drop), I'd never, NEVER spend Y Money on continuing.
Fusing:
You lose the base youkai when FUSING new youkai. Youkai Watch WW/PP is set up in a way that skill levels don't transfer between fusions / evos UNLESS the skill name is exactly the same. So if you're looking to build up Kyuntarou (the pink dood from the first area that makes it easier for youkai to drop), you should only start farming his skill level after you're fusing your Nurse Tongus. If you're only interested in raising the skill level of a fusion youkai, then you should farm exactly ONE of each base youkai, fuse them to get the new fusion youkai (or raise their skill level), then repeat.
You're always trading up when you fuse: i.e. the youkai you sacrifice are lower rank than the youkai you're trying to get. The only youkai that I would hesitate to sacrifice is kyuntarou because he is one of the few youkai that make it easier to collect rarer youkai (though the difference his skill makes isn't that significant.)
One confusing point is that FUSION is handled in a very different manner than the process for unlocking LEGEND youkai. As you collect new youkai, you'll notice that sometimes the game will tell you that you've "unlocked" a certain node. Once you've unlocked all the nodes on a page, you get whatever Legendary Youkai is on that page. As of now, all of the Legendary Youkai (with the sole exception of Bushinyan, the blue Jibanyan dood with the helmet) require one rare youkai only available from the gacha machine. The difference is that you don't lose any of the youkai once you've unlocked a legendary youkai--you only need to have those entries in your youkai album unlocked.
The conditions are
1) only certain combinations of youkai will result in a new youkai
2) the process requires 500 y money
e.g.
ぜっこう蝶 and
ゼッコウ蝶 make
さいこう蝶
As far as I know, there isn't any other way to get さいこう蝶.