Knights of marines MLB or imaginary around?

Use imaginary around for both if you can. MLB Knights of Marine is nice, but Jack has high hits and pretty good NP gain and with Imaginary around she could charge her NP from 0-50 or 60 (couldn't remember), and Okita has slightly higher NP gain and good hits meaning she wouldn't have a problem charging NP too.
Generally use starting NP gain for people who has trouble charging NP like Lancelot berserker and farmers like Fran and Tamacat.
Summertime mistress is good for crit servants like Lancelot and people who has a crit star gather rate skill, but no crit star damage modifier AKA Raikou, Gil, etc.

IA vs Knights depends on the boss and how fast you need to get your NP off. But IA is better in most situations, especially on servants who generate a lot of stars with their quick cards and get their np charge from quick cards (Like Okita and Jack).