There's no optimal combination. I just catch whatever that's available. For me, Pidgey and Ratata very common so those 2 make up the bulk of it. Usually I'll catch enough for 78 evolves before I do it.

My last evolve consists of:

1 Bellsprout	1 Bulbasaur	3 Caterpie
1 Dratini	7 Eevee	1 Ekans
1 Gastly	1 Geodude	1 Goldeen
1 Krabby	1 Nidoran ♂	1 Paras
32 Pidgey	1 Poliwag	2 Psyduck
11 Rattata	1 Slowpoke	1 Spearow
1 Venonat	7 Weedle	2 Zubat

If you can do, let's say, 70 evolve in 30 mins. That's 70000XP. If it's the first time evolve for all of the above 21 Pokemons, you'll get another 21000XP.

Thanks, I missed out too on the 10 coins the first time I beat a gym. Thought have to wait 21hrs before can claim. LOL.
No prob, each evolve will give you back 1 candy.

Results of my 4th Mass Evolve.
76 evolves + 2 new Pokemons = 78000XP