The dojos I'd supose are for min-maxing if you wanna go in deept with the system. I wouldn't bother unless you are struggling with fucking Ninten or something.
You can select the cores and view both the cost and the growth by yourself, but boosting a red 4* with a red 1* core:
Costs 9sp, gains 390xp
with a purple (neutral) 1*
Cost 9sp, gains 300xp
With a gray (support) 1*
9sp, 300xp
Gray, 2*
32sp, 1000xp
With food:
small:
8sp, 250xp
Medium:
32sp, 1000xp
Large:
160sp, 5000xp
I don't wana dismiss any 3 or 4 stars, but if that trend continues, then is more effective using food.
Dismissing spirits actually nets you sp, checking:
4* support:
1 slot 800
2 slots 1000
3 slots 1200
3* support:
1 slot 400
2 slots 500
3 slots 600
2* support:
1 slot 200
2 slots 250
3 slots N/A
1* support:
100sp everyone
–- Update From New Post Merge ---
Didn't check primary spirits as these vary a lot by levels.
--- Update From New Post Merge ---
BTW, don't worry dismissing them before using up for summons, it also pays you automatically.