Training with higher level fighters

You'll lose some net experience by feeding really high level fighters to low level fighters, yes. However, you still do get some bonuses. If I was to take a guess, dropping a level 30, same element, non-same fighter, UC onto a level 1 UC, I'd take a guess that the level one fighter would come out around level 17-18. Or at least that's what it was. Here recently, I haven't seen any change at all. Here in the last couple days, it's seems like GREE has actually removed the bonus for having levels on a consumed fighter.

In truth, I'd actually suggest keeping Citrine. Her stats at Rare are pretty good (8-fighter formula), and even if you don't have eight of them, she drops pretty regularly from both common vans, as well as armored vans. You'll you'll easily be able to get enough to make an 8-fighter version of her. When the next Lightning Epic Boss shows up (probably two weeks from now - since each new epic boss is strong against whatever the last one (Earth > Shadow > Fire > Water > Lightning > Earth), having two maxed out Earth fighters will allow you do do insane damage against the boss. Plus, with two strong units already on your roster, it's probably not the greatest idea to throw one away without a replacement if you participate in the arena.

So, my opinion on the matter: Keep Citrine, and work on Master Jade. Personally, my next to level and max will be Beth, and then after that, I'll probably be maxing a Master Jade myself, while I part time work on Witzig so that I can start swapping out some of my UC's for Rares. It never hurts having an extra strong fighter on your roster. It's not like your roster ever really gets "full", right?

Will gain more attack by fusion a uc+ with a uc then fusion a uc+with a uc+

For UC's, you'll (almost) always get 20% of the lost units stats.

For example, if the unit your fusing has 100 ATK, you'll get +20 attack after fusing.

500 attack? +100 to your fighter. 1000 attack? +200 to your fighter. And so on.

When working with UC's, you can actually use a calculator to figure out what the stats will be after fusion (current value x 1.2 = after fusion vaue (round up - 253.1 becomes 254 for example). Comes in handy with figuring out the total values for fighter stats.

Thanks for a great answer! So it's also probably better (if thinking fighter-efficiency) to train my main fighter with 8 commons rather than putting those 8 into a lvl 1 U and feed that to the main fighter. Guessing this is only for money-efficiency? (Read your great guide :))