I did some math on leveling up, cost of food/fodder heroes, and chance to increase special…
BEST odds are using same color 1*'s
Next best are using same color 2*s. (2* feeders have twice the chance, but give more than twice the experience, so you get to use less than half as many, so chances are actually lower.
Using 3* feeders is worse odds, 4*'s are even worse still, and Trainer Heroes are even worse odds of increasing special than an off-color 1* or 2*! (Because you get so many XP)
I also did the math on leveling up assuming an average of 7 1* and 3 2* (Maybe 50/50 is more realistic? I dunno…) and calculated the EXPECTED number of special increases, given how much experience you need to get to various places:
5*, 2/60 – 7.4 expected for 9/8 – in other words, MOST 5* heroes will hit 8/8 before they need ascension items, 'tho if you have bad luck you might not quite.
5*, 3/70 – 17.7 expected – in other words, luck so bad as to not get there doesn’t really exist!
In fact, you have half the odds (roughly) if you feed him a rainbow, and taking a 5* to 3/70 is still almost certain to get 8/8 special.
Things are better for your 4*/60, almost certain to be 8/8…
And it is tighter for your 3*/50, expected special is 7.9/8 … in other words half the time leveling with that mix you won’t get all the way there.
If you are really concerned, give all your 1*s to somebody still working on their special…and your 2*s and especially trainers to somebody who is at 8/8.
Also, note, that food used to level up is double at level 51 what it is at level 1. So using your trainers for the ones at high levels will save you food.